But are there any other 'in-universe' explanations to the long life of the TOS crew such as in my original post ?
I don't see the need. McCoy's age of 137 in "Farpoint" and Jadzia's prediction in one episode that O'Brien would live to 140 suggest that that's a fairly typical life expectancy for humans in the 24th century, or toward the upper limit of life expectancy. Since O'Brien was included, it's clearly not limited to the TOS cast; and it's simple enough to conclude that future geriatric medicine is sufficient to explain the longevity increase.
Besides, the only canonical example we have of a human TOS character living that long is McCoy, plus Uhura and Chekov in the prose. That's at most three out of 400-odd crewmembers (considerably more if you count all the various comings and goings over the years). Okay, if you limit it to the core command crew that stuck together for decades, that's fully half the human population of the core group, a bit less if you count Chapel. Three out of a random
sampling of six or seven living close to the maximum life expectancy (if it is maximum rather than average) would be anomalous, but this is three members of a closely associated group, so it's logical that they would've had common lifestyle factors affecting their longevity. As Starfleet officers they'd all stay in excellent physical condition, and they'd all have access to the finest medical care. The only problem is that, as Starfleet officers, they'd also be exposed to frequent danger, injury, radiation exposure, alien diseases and toxins, and other factors that would tend to shorten their life expectancy, statistically speaking. But if they managed to survive their years of active duty and ultimately end up in desk jobs, then their survival for decades beyond that is not unlikely if they maintained the good fitness habits they'd acquired and continued to have access to the finest medical care -- both reasonable expectations if they remained in Starfleet.