Well, I wouldn't give him sole credit, since he didn't conceive the stories to those films. Anyway, those were both cases of resetting things to real time after the previous installments had compressed time. TMP was made a decade after TOS and pretended to be only a few years later. And TWOK through TFF were made over the course of seven years but spanned less than a year of story time (despite the Okudachron's clumsy effort to spread them over three years). I think that, in both cases, the makers of TWOK and TUC just decided "Okay, we can't keep pretending the actors are that young anymore."