The phrase "median lifespan" means the average age at which half of a group of people or animals will be alive and the other half will have died. Full definition
25 % increase in median lifespan, don't get me wrong. (fightaging.org)
@Misiu: This is a result about median lifespan of a population, nothing to do with the lifespan of a single individual. (fightaging.org)
[7] A UK Kennel Club survey puts the breed's median lifespan at 12 years 11 months, with about 1 in 5 dogs dying of old age at an average of 14 — 15 years. (en.wikipedia.org)