A bit off-topic but here's something I noticed a while ago, maybe others have noticed too.. The TOS crew seemed to do a lot of time travelling. Not just that, but they seem to keep losing time as they time travel.
After the events of "The Naked Time" they're subjectively or biologically two or three days older than their chonological age. In COTEOF, Kirk and Spok aparently spent between a week and a month, McCoy less, in the past and Scotty said they were only gone for a moment. Once again they're biologically older than their chonological years.
In Star Trek IV, same thing, the crew appeared to be gone only a moment when they time travelled.
Man, when Kirk reached his 50th birthday he must've looked and actually was 80. But it may explain a bit. I've heard the events of TMP took place maybe about 3 years after the events of TOS yet the crew looks about ten years older. Now you know why.