You get older every day. But how old are your cells and organs, really? Understanding that could help us live longer, healthier lives, many longevity experts believe. Dr. Douglas Vaughan is among them ...

LIKE MANY OF you, especially those over 40, I envision myself as younger than my chronological age. Subjective age—how old you feel—is a thing, and people over 40 on average feel 20 percent younger.