Near the start of the 1969 film ‘Butch Cassidy and the Sundance Kid’ is the memorable line, “I’m over the hill but it can happen to you. Every day you get older, now that’s a law.”
The rate at which you add birthdays is constant. One every year. Those birthdays add up to your chronological age.
The rate at which your body ages is variable. It is up to you. The decisions you make and the actions you take determine how quickly or slowly your body ages. If you do drugs, smoke, drink alcohol, consume a diet of processed foods and don’t exercise you age much faster compared to the clock. Your biological age will be older than your chronological age.
Biological age is a more accurate measure of healthspan, or years lived in good health, than chronological age, and doesn’t directly correlate with wrinkles and gray hairs. Rapid agers experience a faster rate of functional deterioration relative to their chronological age.
There is a good amount of data on it at: https://thewellnesswatchdog.com/biological-vs-chronological-age/
Here’s a simple biological age calculator: https://bioagecalculator.com/ This one gives mine at 4 years less than my 72 years.
This one https://calculator-online.net/biological-age-calculator/ has double the questions and pegs my chronological age at 54.6 and predicts I’ll live to 103.