Maybe he was secretly a smoker, I heard that ages you.Except that the rate of aging was inconsistent, and seemed to parallel Sarek's aging process. Both of them seemed to age 20 human years between TOS and the later movies, then they didn't age at all until TNG. In the years since TNG Spock appears to have aged at a much quicker rate again. I guess Sarek had some genetic abnormalities. I can only guess that with Sarek it's better explained that he's just a generation older than Spock so his aging isn't quite as pronounced, though I'd think his disease would have aged him quite a bit. Also, I think that for Spock diplomacy must have seemed far less stressful than his career in Starfleet or his attempts at unification on Romulus.Any aging issues with Spock the character can be explained at least in part by his half-human DNA and lineage. Chronologically, Spock was only 62 or 63 years old in THE UNDISCOVERED COUNTRY but looked a bit older and much more haggard...especially when you consider that Tuvok on VOYAGER was 109 years old at the time of the 24th century events in "Flashback" but looked closer to thirty or forty.Tuvok was a full-blooded Vulcan(so far as anyone ever knew) while Spock wasn't. Hence he aged more like a Terran than most Vulcans.