'Aging clocks' tell you how much 'older' you are than your chronological age. How do they work?

If you want to know your chronological age, simply count the candles on your next birthday cake. Calculating your biological age, though, is a little more complicated. Chronological age is the number of years between your birth and now; it's purely time-based. Biological age, on the other hand, describes the progressive breakdown of an individual's physiological and molecular systems over time; it's a measure of how "aged" the body is. The calculation aims to answer the question of how well your systems, organs and cells are working compared to an average, healthy baseline. "Biological age is notoriously hard to define...