i wonder why they felt they had to set it so soon after TOS? Why not 10 years as in real time to account for the actors aging and radical redesigning..?
they got it right for TWOK moving forward a few years...(as did VI i think)
(similarly STID was also surprisingly soon after ST09 yet had the 2nd longest gap inbetween films)
I read somewhere I think that Shatner was at that time barely accepting he was forty. So maybe they didn't want to aggravate him by saying more (actual) time had passed.
However it could be that was in TWOK not TMP. Can't remember who said it (Meyer or Shatner?).