Today, scientists distinguish four generations of “aging chronometers” that assess age-related changes in the body using various parameters. (Image: ktsimage via Getty Images)
Chronological years are easy to determine by the number of years lived, but biological age requires more complex calculations.
Chronological age reflects the time interval from birth to the present. Biological age characterizes the gradual deterioration of the molecular and physiological systems of the body, showing how intensively it wears out. The main goal of such measurements is to compare the functional state of human organs and cells with the average statistically healthy indicators.
“Biological age is difficult to formalize because it’s conceptual,” says Eric Sun, an associate professor of biomedical engineering at MIT who plans to open his own lab in 2026. The concept, he says, focuses not on years lived but on the body’s functional capabilities and potential risks for developing diseases.
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Experts have created several types of “chronometers” that assess biological age. Let's look at their operating principles and possible benefits.
What is the biological clock of aging?
Just like a battery that loses capacity over time, the human body gradually reduces its functional reserves. The tools for measuring these reserves are called ohmic clocks or biological age tests.
The science of biological age continues to evolve. The first models of such watches appeared in 2013. Today, there are dozens of options that analyze protein profiles, immune responses, and epigenetic changes — DNA modifications that affect gene activity without changing their structure.
How do aging chronometers work?
Most models are based on machine learning algorithms that identify patterns in data. Regression allows you to predict the probability of events by taking into account the importance of various factors (weighting coefficients). For example, when assessing the risk of lung cancer, smoking is given a high weight, and radon exposure is given a lower weight.
Algorithms are trained on thousands of biomarkers — indicators that reflect the state of the body. Blood analysis is especially informative, since blood interacts with all organs, revealing signs of disorders.
After processing the data, the algorithm generates rules for interpreting new samples, allowing it to predict a person’s health without knowing their exact age.
Epigenetic clock: how it works
The first models analyzed DNA methylation — the addition of methyl groups to specific areas of the genome, regulating gene activity. Methylation patterns change with age, allowing biological age to be estimated. The difference between actual and predicted age (delta) indicates the rate of aging.
A 2024 study describes four generations of epigenetic clocks:
1. First generation: evaluates age delta only.
2. Second generation (e.g. PhenoAge): predicts mortality and disease risks using organ function data.
3. Third generation (DunedinPACE): measures the rate of aging, like a speedometer.
4. Fourth generation: identifies causal relationships between methylation and age-related changes using the Mendelian randomization method.
Alternative types of chronometers
In addition to epigenetic ones, there are:
• Proteomic clock: analyzes protein profiles to find therapeutic targets.
• Metabolic clock: studies metabolites, which is convenient for large-scale studies.
• Transcriptome clock: monitors gene activity through RNA.
• Multi-ohm clocks: combine different approaches.
Practical application
The usefulness of the watch depends on its ability to predict risks and respond to therapy, says Harvard's Dr. Dan Henderson. Sun believes that the main application for now is in research, where the tools can help quickly assess the effects of interventions.
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Experts agree that current models are not yet ready for clinical use. The risk of erroneous predictions and incorrect conclusions remains high, which can lead to unjustified treatment.
Sun envisions that future personalized models will analyze multiple biomarkers at the organ level.
Disclaimer
The material is for informational purposes only and does not replace specialist consultation.
Patrick Sullivan, Live Science contributor
Professional journalist specializing in medical topics since 2015. Lives in New Jersey, enjoys yoga and Brazilian jiu-jitsu.
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