Chronological age is the number of years, months, days, etc. that have passed since a person was born. It is the primary way people define their age and is a primary risk factor for chronic diseases, mortality, and any impairments to bodily functions, such as hearing and memory. Biological age, on the other hand, is determined based on biological and physiological development factors. It takes into consideration a number of factors other than just the day a person was born, such as the gradual accumulation of damage to various cells and tissues in the body. Biological age is also known as physiological or functional age.

The difference between chronological and biological age can be surprising and definitely worthy of further exploration. While chronological age is always an easy-to-determine number, biological age depends on a number of variables that can change on a continuing basis. For example, a persons chronological age may be 70, but their biological age may be 50 if they have good physical conditioning and are functioning well.

In some contexts, chronological age might give a useful approximation of those characteristics that matter, but from the point of justice, it is far from clear that it is so in every context or in every specific case. The distinction between biological and chronological age could have significant implications on how we should understand legal age. Currently, legal age always corresponds with chronological age, but the results from research into epigenetics imply that it may soon be possible to use biological age as a base for claims of legal age change.