Lifespan

Lifespan is how long an individual lives, and in research it is reported in at least three different ways that are easy to mistake for one another.

Median lifespan is the age by which half the group has died. Maximum lifespan describes the oldest survivors, and because a handful of animals is a fragile basis for a number, it is often reported instead as the age at which 90 percent of the group has died. Mean lifespan is the average, which a few very early deaths can drag down without telling you anything about aging.

The distinction changes what a result means. A treatment that lifts median lifespan without touching the maximum has postponed common causes of death. A treatment that lifts the maximum has done something to the ceiling. The 2009 study that established rapamycin as a life-extending compound in mice reported both, and gave its headline figures — 14 percent in females, 9 percent in males — at the age of 90 percent mortality.

Lifespan is also distinct from healthspan, and the gap between them is the part most people care about. Years added at the end of life are not automatically years worth having, and a study reporting one of these is not reporting the other.