Why did the cast age so much between TMP and WOK?

[brightspot]

As you age, your skin becomes thinner and lighter (less pigment and fat remains in our epidermis). Sometimes people who are older try to mimic the color of their hair when they were younger, using hair dye, but it seems artificial - their skin is actually a shade or two lighter than it was thirty years before. The costumes in TMP, in the pastel tones used, are more flattering for the original cast, but the dark magenta was too harsh, especially on Shatner and Kelley.

 

[Steve Gennarelli]

Harve Bennett has said that in his opinion, they tried to look "young" when they filmed "ST: TMP". They embraced the aging process in ST: TWOK.
I've always felt Bill looked younger in Trek's II and III than he had in the 1st feature film. By 3, he actually had a tan from filming "TJ Hooker" on the streets of LA. His hair was much more flattering to his face in 2 than in 1. The uniforms for II-VI also benefited the cast.
Leonard actually looks younger in Trek IV as he's out or about out of a difficult marriage by then and he had gotten himself into Spartan shape for directing and starring in IV. I think by that time he had quit drinking and smoking and that had improved his skin.