Mandatory retirement

From Memory Alpha, the free Star Trek reference.

Until at least 2270, the Federation Starfleet had a policy of mandatory retirement set (for Humans) at age 75.

When in 2270 Commodore Robert April proved that he was still very much capable of commanding a starship at 75, Starfleet reviewed his appeal to have his retirement delayed. (TAS: "The Counter-Clock Incident") The policy was apparently rescinded by the time of the launch of the Enterprise-D, as Leonard McCoy was still a Starfleet admiral at age 137. (TNG: "Encounter at Farpoint")