Mandatory retirement

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. The policy was apparently rescinded by the time of the launch of the USS Enterprise (NCC-1701-D), as Leonard McCoy was still a Starfleet admiral at age 137.