The Deadly Years

Enterprise discovers the scientists on Gamma Hydra IV are all either dead or dying of old age, despite the fact that the oldest was around thirty! Matters get worse when the landing party begins to show signs of aging, all except for Chekov, who is mysteriously unaffected. Eventually, Kirk must be relieved of command, and as Spock and Scotty are both affected, Commodore Stocker assumes command.

Synopsis adapted from TVmaze (CC BY-SA 4.0)