Didn't one of the Admirals in one of the early episodes say he was approaching "retirement age"? He must have been 70 or so.
Robert April said so in TAS. Even ignoring the questionable canonicity of TAS, the end of the episode does say Starfleet is reviewing the mandatory retirement policy.
Anyway, I always assumed that in the 24th century while it is common to live past 100, making it to 130 or 140 or however old McCoy was supposed to be in Farpoint is the equivelant of making it to 100 today, it happens but it's rare.
If you want to include novels, the novels actually have all TOS characters still alive and active in the 2370s and 80s.