Aging is a complicated process and is highly varied among individuals. We all have that friend who "doesn't look a day over 30" well into middle-age, and the less fortunate friend who isn't aging "gracefully". You may notice aging variability within your own body. Perhaps you have overall excellent health, but your digestion has slowed or you have sleep-pattern changes. Research shows that the quantity of years is a poor estimator as an index of the aging process. Even twins often have different biological or functional ages, and within an individual, organs may age at different rates.
Identifying a method to accurately estimate biological age is important for predicting the onset and prognosis of diseases, improving life quality, understanding the mechanism of aging, and addressing factors that lead to the decrease of function. Currently, there is no single golden index to effectively replace chronological age. The article titled: “Common methods of biological age estimation” systematically reviews, summarizes, and compares the four most common biological age estimation methods used in longevity studies today.