Well the Trek Lit writers seem to assume that McCoy's lifespan is entirely typical, what with Picard being a frontline commander in his 80s who sees nothing problematic about having a child at that late age, and Vaughn being over a hundred and still kicking ass. And of course, Losing the Peace
establishes that McCoy's still a practicing doctor more than fifteen years after "Encounter at Farpoint".
On the other hand, such longevity isn't compatible with the TOS era where Kirk was complaining about getting old when he was still in his 40s. The only logical conclusion is that there were some major medical breakthroughs in the late 2200s -- although even that's problematic since "Too Short a Season" shows Admiral Jameson as a doddering old man in his 70s.