History of biological age tests («age clocks»)
In 2013 some researches realized that our epigenome is constantly changing and that this change correlates with age. By using machine learning on many epigenetic samples to predict chronological age, the first generation of biological age tests were born.
With these clocks, a result, e.g., 40, means you are epigenetically similar to an average person of age 40.
However, chronological age does not account for the biochemistry of our bodies. This is why some 50 year old people look like they're 30 and vice versa. To account for these differences, i.e., your underlying health, second generation clocks were developed.
With these clocks, a result, e.g., 40, means you have a similar disease risk to an average person of age 40.
Despite the improvement, second generation clocks still had shortcomings. These clocks still cannot tell you if you are currently living a healthy life. A historically unhealthy person that has recently started with healthy habits and a historically healthy person that recently let themself go could have the same biological age signature. This is why the third generation of clocks, the pace of aging, was developed. There is currently just one pace of aging clock, called DunedinPACE.
It was developed by following the population cohort of Dunedin born in 1972/73 since their birth until now. DunedinPACE is the most precise clock with the best risk prediction profile currently available. With these clocks, a result, e.g. 0.8, means you currently age 0.8 biological years per calendar year.