While you can see some external signs of aging (gray hair, wrinkles), biological aging largely happens invisibly inside your cells. Biological age is how old your cells are and how various diseases and conditions are present (or absent) in your body.

Your biological age is a trickier to determine. We all celebrate a birthday once a year, meaning everyone’s chronological age increases at the same rate. Your chronological age is the one you share (or fudge a little) with others: It’s how many years you’ve been alive. Fortunately, there are things you can do right now to make that happen and add health to your years. If you’re looking to optimize your health and ward off age-related diseases, you want your biological age to be at least equal to-but ideally lower than-your chronological age. The former can be measured by ripping days off the calendar the latter by measuring biomarkers that are linked with getting older, including changes to your DNA and cells. Your biological age, on the other hand, is how well your body is holding up compared to others with a similar birth date. Your chronological age is the age you’re familiar with in simple terms it’s how many birthdays you’ve checked off. Sure, age might be nothing but a number, but which number? You actually have two ages: your chronological age and your biological age-and odds are good that they’re not the same.

Biological and chronological age