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Dimensions of Chronological and Biological Age

Dimensions of Chronological and Biological Age

Contributor Bio

Dr. Zoya Marinova is a researcher and medical writer. Her research interests are in the fields of epigenetics and molecular neurobiology. She received her PhD in Experimental Neuroscience from Karolinska Institute. Dr. Marinova has co-authored publications in renowned journals, such as Molecular Psychiatry and Journal of Neuroscience. She has worked on basic, translational, and clinical research projects investigating the molecular mechanisms of CNS disorders.

Chronological and Biological Age

Aging is a universal process that has been defined in many different ways. A broad definition states that aging is the “progressive loss of physiological (normal functional) integrity, leading to impaired function and increased vulnerability to death.”1 Aging profoundly affects every living organism. Moreover, since the human lifespan has considerably increased throughout the last two centuries, the number of elderly people worldwide is also rising. In 2019, 703 million persons globally were at least 65 years old. Moreover, the number of people aged 65 years or above worldwide has increased from 6% in 1990 to 9% in 2019.2

Chronological age is defined by the time that has elapsed from the birth of an individual until the point in time when the age is assessed. However, the speed with which people age varies. Whereas some people may suffer from age-related disorders in their 60s, others may not have substantial age-related problems at the same age. Therefore, the term biological age has been introduced, which reflects the biological process of aging rather than just the chronological passing of time.

Normal aging is associated with gradual changes, such as a decline in hearing, visual acuity (the ability of the eyes to discriminate details), muscle mass, and immune function.3 Moreover, the prevalence of many medical conditions increases with age. Such conditions include cardiovascular diseases (diseases affecting the heart or blood vessels), diabetes mellitus (a disorder associated with increased blood glucose levels), disorders affecting the skeletal system, and medical conditions affecting cognition.3

Due to the universal significance of the aging process, the biological mechanisms that underlie it have been the subject of active research. Both genetic factors and environmental factors/lifestyle factors affect the aging process. The contribution of genetic factors to life expectancy is complex, but one study estimated it to be approximately 25%.4 However, environmental and lifestyle factors also play a role. For example, healthy eating, avoiding excessive alcohol consumption, avoiding tobacco use, and maintaining physical activity are all important to remain healthy in older age.4

Biological Parameters Associated With Age

Different approaches have been developed for the assessment of biological age. They include biomarkers or the assessment of deficits or frailty.5 Biomarkers are naturally occurring in the body characteristics or chemical compounds that help identify certain physiological (healthy) processes or medical conditions. Frailty is most commonly defined as a combination of signs indicative of health decline that are associated with negative health outcomes.6 Such negative health outcomes include falls, more frequent need for hospital stays, and mortality risk.

Lopez-Otin et al. (2013) postulated nine biological parameters correlated with aging.1 They include:

  • Cellular senescence (the process of cellular aging)

  • Stem cell exhaustion (the decline of function of unspecialized cells that divide throughout life and give rise to cells that can undergo specialization)

  • Changes in intercellular interactions

  • Genomic instability (proneness to changes in the genetic information of an organism)

  • Epigenetic modifications (heritable changes in gene function that are not due to changes of the DNA sequence)

  • Telomere attrition (shortening of the ends of the chromosomes)

  • Mitochondrial dysfunction (dysfunction of the cellular compartments responsible for cellular breathing and energy production)

  • Loss of proteostasis (loss of the dynamic regulation of proteins)

  • Deregulated nutrient sensing (deregulated sensing of substances needed for health) 

DNA Methylation

Epigenetics assesses how the environment and behavior cause modifications that affect the way in which genes function.7 Epigenetic alterations have shown especially high association with aging.8 DNA methylation is an epigenetic mechanism implicated both in normal cell function and in disease processes. DNA is a complex molecule that contains the genetic information needed to direct the activities of living organisms.9 DNA molecules are composed of two paired, twisted strands consisting of four chemical units that are also known as nucleotide bases: adenine, cytosine, guanine, and thymine. DNA methylation represents the addition of a methyl group to a specific position in the nucleotide base cytosine, forming 5-methylcytosine. DNA methylation has been identified predominantly on cytosines followed by guanines, which are known as CpG sites. 

DNA Methylation Throughout Aging and Epigenetic Clocks

A number of studies have investigated the association between DNA methylation and aging. The association of DNA methylation in a set of CpG sites and aging has allowed the development of tools that can predict age through the investigation of the DNA methylation status, which are also known as epigenetic clocks.10

Some of the most well-known epigenetic clocks include Horvath’s clock, Hannum’s clock, and Levine’s clock. Hannum’s epigenetic clock was developed for use with blood samples and is based on 71 CpGs. Its correlation with chronological age is estimated to be 96%, and its findings are affected by genetic factors and gender.11 Horvath’s epigenetic clock is based on 353 CpGs and can be used on a wide range and cell types.12 It also is highly correlated with the chronological age. 

Differences in DNA Methylation Across Cell Types

The patterns of DNA methylation of genes may differ among cell types. However, some of the epigenetic clocks, such as Horvath’s clock, have been developed to function across different types of cells. Horvath’s epigenetic clock can identify the same consistent DNA methylation age across different cell types in the body. Notably, the age of stem cells and the age of cancer cells differ markedly from the age of other cells. The DNA methylation age of stem cells determined with Horvath’s epigenetic clock was close to zero. Contrarily, cancer cells from different cancer types consistently showed accelerated DNA methylation age, with an average age acceleration of 36.2 years.12 

Association of DNA Methylation Age With Mortality Risk, Age-Related Medical Disorders, and Lifestyle Factors

The difference between chronological age and DNA methylation age is considered to provide information about the process of biological aging. A study assessed the difference between DNA methylation age determined with the Hannum’s and Horvath’s epigenetic clocks and chronological age and found that accelerated DNA methylation age is associated with an increased mortality risk.13 In the same study, a 5-year DNA methylation age acceleration according to Hannum’s epigenetic clock was associated with a 21% increase in the mortality risk, whereas a 5-year DNA methylation age acceleration according to Horvath’s clock was associated with an 11% increase ointhe mortality risk.13 DNA methylation age acceleration has also been observed in patients with neurodegenerative disorders, such as Alzheimer’s disease14 and Parkinson’s disease.15 A study comparing blood cells of patients with Parkinson’s disease and healthy individuals found that the DNA methylation age in patients with Parkinson’s disease was accelerated by 1.5 years.15 Both Horvath’s and Hannum’s clocks-associated DNA methylation age acceleration have also been correlated with indicators of metabolic syndrome.16 Further, DNA methylation measures based on Horvath’s and Hannum’s epigenetic clocks have also been associated with certain lifestyle and environmental factors, although the association has generally been weak. However, while cumulative lifetime stress has been associated with accelerated DNA methylation age, such a correlation has not been established for acute physiological stress.10 Notably, the processes underlying the DNA methylation age acceleration have also been found to be under substantial genetic control.10 

Development of an Epigenetic Clock Incorporating Clinical Characteristics

Levine at al. developed an epigenetic clock that assesses changes in DNA methylation markers in association with a complex proxy measure of physiological dysregulation (designated also “phenotypic age”) rather than with chronological age alone. The authors used a combination of 10 clinical characteristics, one of which was chronological age, whereas the remaining nine were molecular or cellular parameters. This optimized age estimator demonstrates a higher correlation with and predictive ability for negative health outcomes17 than estimators considering only chronological age and captures the gap between chronological age and phenotypic age. Overall, 513 CpGs have been included in Levine’s epigenetic clock, and they only partially overlap with CpGs used in other biological clocks. Levine’s epigenetic clock includes CpGs that strongly correlate not only with age but also with the risk of shortened lifespan and increased disease prevalence. DNA methylation age acceleration determined with the Levine’s epigenetic clock is also correlated with an increased risk of heart disease17 as well as with an elevated body mass index (a measure of a person being at a normal weight, overweight, or underweight), smoking, and alcohol drinking.18

Parameters that have been associated with accelerated DNA methylation age across Hannum’s, Horvath’s, and Levine’s epigenetic clocks include cancer, mortality risk, Parkinson’s disease, and blood levels of C-reactive protein (an indicator of inflammation), glucose (blood sugar), insulin, and triglycerides (a type of fat in the body).10 Interestingly, gender has also been associated with differences in DNA methylation age. Higher epigenetic age has been observed in boys than girls, which may possibly be attributed to the higher birthweight of boys.10 

Association Between DNA Methylation Age and Frailty

As mentioned earlier, frailty is a combination of signs indicative of health decline associated with negative health outcomes.6 Frailty has been defined both in terms of physical parameters and of broader deficits.19 The physical parameters analyzed in the context of frailty include decreased walking speed, extreme tiredness, decreased physical activity, weight loss, and weak hand grip. The frailty-related deficits are defined in broader terms of clinical signs, medical conditions, and disability. Accelerated DNA methylation age has been associated with an elevated likelihood of being physically frail; however, this correlation is not universal to all DNA methylation age measures.19 DNA methylation age acceleration has also been associated with a comprehensive frailty measure; in the same study, 6 years of methylation age acceleration were associated with half of an additional deficit included in the comprehensive frailty measure.20 The identified association between DNA methylation age measures and frailty supports the biological significance of the DNA methylation age. 

Complexity of the Aging Process and Its Biological Correlates

The aging process is not only universal but also complex, and it progresses at variable speeds between individuals. Thus, discrepancies between the biological age and chronological age may arise. Some of the approaches developed to capture aspects of the biological age include epigenetic alterations and frailty. Several epigenetic clocks have been established that assess DNA methylation changes in CpG sites sets correlated with chronological and phenotypical age. Associations have also been identified between DNA methylation age and frailty measures. Even though these measures cannot replace the clinical evaluation of the aging process, they can help gain valuable insight into the mechanisms implicated in biological age.



  1. López-Otín, C., Blasco, M.A., Partridge, L., Serrano, M., & Kroemer, G. (2013). The hallmarks of aging. Cell,153(6), 1194–1217.


  3. Jaul, E., & Barron, J. (2017). Age-related diseases and clinical and public health implications for the 85 years old and over population. Frontiers in Public Health,5, 335.


  5. Jazwinski, S.M., & Kim, S. (2019). Examination of the dimensions of biological age. Frontiers in Genetics,10, 263.

  6. Xue, Q.L. (2011). The frailty syndrome: Definition and natural history. Clinics in Geriatric Medicine,27(1), 1–15.


  8. Jylhävä, J., Pedersen, N.L., & Hägg, S. (2017). Biological age predictors. EBioMedicine,21, 29–36.


  10. Horvath, S., & Raj, K. (2018). DNA methylation-based biomarkers and the epigenetic clock theory of ageing. Nature Reviews Genetics,19, 371–384.

  11. Hannum, G., Guinney, J., Zhao, L., Zhang, L., Hughes, G., Sadda, S., Klotzle, B., Bibikova, M., Fan, J.B., Gao, Y., Deconde, R., Chen, M., Rajapakse, I., Friend, S., Ideker, T., & Zhang, K. (2013) Genome-wide methylation profiles reveal quantitative views of human aging rates. Mol Cell49, 359–367.

  12. Horvath, S. (2013). DNA methylation age of human tissues and cell types. Genome Biology,14(10), R115.

  13. Marioni, R.E., Shah, S., McRae, A.F., Chen, B.H., Colicino, E., Harris, S.E., Gibson, J., Henders, A.K., Redmond, P., Cox, S.R., Pattie, A., Corley, J., Murphy, L., Martin, N.G., Montgomery, G.W., Feinberg, A.P., Fallin, M.D., Multhaup, M.L., Jaffe, A.E., Joehanes, R., Schwartz, J., Just, A.C., Lunetta, K.L., Murabito, J.M., Starr, J.M., Horvath, S., Baccarelli, A.A., Levy, D., Visscher, P.M., Wray, N.R., & Deary, I.J. (2015). DNA methylation age of blood predicts all-cause mortality in later life. Genome Biology 16(1), 25.

  14. Levine, M.E., Lu, A.T., Bennett, D.A., & Horvath, S. (2015). Epigenetic age of the pre-frontal cortex is associated with neuritic plaques, amyloid load, and Alzheimer's disease related cognitive functioning. Aging (Albany NY),7(12):1198–1211.

  15. Horvath, S., & Ritz, B.R. (2015). Increased epigenetic age and granulocyte counts in the blood of Parkinson's disease patients. Aging (Albany NY), 7(12), 1130–1142.

  16. Quach, A., Levine, M.E., Tanaka, T., Lu, A.T., Chen, B.H., Ferrucci, L., Ritz, B., Bandinelli, S., Neuhouser, M.L., Beasley, J.M., Snetselaar, L., Wallace, R.B., Tsao, P.S., Absher, D., Assimes, T.L., Stewart, J.D., Li, Y., Hou, L., Baccarelli, A.A., Whitsel, E.A., & Horvath, S. (2017). Epigenetic clock analysis of diet, exercise, education, and lifestyle factors. Aging (Albany NY),9(2), 419–446. doi: 10.18632/aging.101168

  17. Levine, M.E., Lu, A.T., Quach, A., Chen, B.H., Assimes, T.L., Bandinelli, S., Hou, L., Baccarelli, A.A., Stewart, J.D., Li, Y., Whitsel, E.A., Wilson, J.G., Reiner, A.P., Aviv, A., Lohman, K., Liu, Y., Ferrucci, L., & Horvath, S. (2018). An epigenetic biomarker of aging for lifespan and healthspan. Aging (Albany NY)., 10(4):573–591.

  18. Chen, M., Wong, E.M., Nguyen, T.L., Dite, G.S., Stone, J., Dugué, P.A., Giles, G.G., Southey, M.C., Milne, R.L., Hopper, J.L., & Li, S. (2019). DNA methylation-based biological age, genome-wide average DNA methylation, and conventional breast cancer risk factors. Scientific Reports,9(1), 15055.

  19. Gale, C.R., Marioni, R.E., Harris, S.E., Starr, J.M., & Deary, I.J. (2018 ). DNA methylation and the epigenetic clock in relation to physical frailty in older people: the Lothian Birth Cohort 1936. Clinical Epigenetics, 10(1), 101.

  20. Breitling, L.P., Saum, K.U., Perna, L., Schöttker, B., Holleczek, B., & Brenner, H. (2016). Frailty is associated with the epigenetic clock but not with telomere length in a German cohort. Clinical Epigenetics,8, 21. doi:10.1186/s13148-016-0186-5