What in the world for? Vulcans have to be 34 years old some time. Do you think they sit in a coccoon until they're 60?
Because Nimoy is still being employed in the role of Spock almost a century later in TNG and now post-TNG IF Spock is 34 in 2264, and then he's 63 in 2293 (STVI) then he is aging, over that 30 years, like a human...then he's basically twice that age in Unification - something that was filmed BEFORE STVI....and in that second 60 years he's aged not at all? To say nothing of the fact that babealicious T'Pol was said to be in her 60's and Tuvok was said to be over 100. You lose nothing at all if Spock is 50 or 60 during TOS in any way that matters and the on screen aging between, for instance, STII and Unification - roughly 70 fictional years - is a lot more natural.
Maybe I'm crazy, but I think that we have to suspend disbelief when it comes to the relative ageing of Nimoy and Spock.
Spock's age is gonna be all fucked up anyway. First off, he is half-human so he'd probably age quicker than T'Pol or Tuvok or any of the other pure Vulcans. Then he was afflicted by a rapid aging disease in 'The Deadly Years.' He had his brain removed and re-inserted in 'Spock's Brain.' His physiology reverted to some primordial Vulcan state in 'All Our Yesterdays.' He died from radiation in Star Trek II. Then his body was caught up in the matrix of a new planet and he regenerated as a child Then he started rapidly aging AGAIN! (after having sex with Saavik) And there was all that time travelling. Good luck trying to figure out how old he really is...