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.

Your head is actually aging faster than the rest of you. Before you freak out, though, know that it’s only by nanoseconds—90 billionths of a second over 79 years, to be exact. That kind of difference ...