It was discussed a while ago in the Enterprise relaunch thread, re the possibility of Archer living into the TOS era and it was (kind of) agreed that top end life expectancy as established by McCoy in TNG was around 150 years. In the real world I would expect that to be on the low side for the 23rd / 24th centuries, but this is about the Trekverse.
In the novels it has been established that all of the main TOS crew have survived (except Sulu ?) into the TNG era and beyond, most going the 'long' route and living through each year.
I would think it statistically unlikely that any non related group of people would all be naturally long lived and, having been watching reruns of TOS wondered if they have a common experience that would explain it.
In 'Miri' there could have been a 'leftover' effect of the disease that extended lifespan, but not all of the command crew were exposed. However, in 'This Side Of Paradise' the whole crew is exposed to the spores which as a side effect grant perfect health. This would have two effects - firstly to reset cellular degeneration back to that of an infant : the thirty something crew would effectively be zero years old again, and secondly, with perfect health as a starting point (which is near-impossible in nature) a very long life free of any disease other than those developed through infection seems likely.
What do you think ? Are the TOS crew all unusually long lived, and would that explain it ? Are there any events in other episodes that may be responsible ?