Actors who seemed to age overnight?

William Holden

Hell yes. I just seen William Holden in Irwin Allen's When Time Ran Out...(1980) He looked like hell. His romantic scene with Jacqueline Bisset had me doing a double take, he looked like her grandpa. He looked way older than his years thanks to his heavy drinking. One of the great actors though.

Mickey Rourke was on Celebrity Big Brother here and the way I gasped. No idea if that was more of a gradual thing though.

Jamie Lee Curtis

James Stewart seems to either be like a spring chicken Phil Mickelson, or a crusty old fart.

He was too old for many of his roles in the 1950s but managed to pull if off.

The problem with aging well, with appearing to be ageless for a decade or two, with "winning" the evergreen Olympics, is that the bill eventually comes due and your ass is going to go Dorian Gray with a quickness. It's called "the wall." The longer you stretch it out, the harder you hit it. Pick your poison. Age gracefully, fade, see people cringe a little as you slowly age out, or cheat for a more time, but hear people gasp with audible shock when the Jenga tower finally collapses and the Botox, surgery, exercise, make-up, steroids, etc., no longer work their glamor.

David Krumholtz

Might be all the weed he smoked.

He didn't look too great in Network either but he still gave the best performance in the film.

There's no hope for us anymore

I read William Holden was repeadely drunk on the set of When Time Ran Out...(1980) so much so they had to hospitalize him for six days trying to get him to dry out.

I've heard he was right up there with Oliver Reed as far as Hollywood's greatest drunks were concerned.

I'd forgotten, but I think I'm right in saying Holden died when he tripped over something at home drunk.
May be wrong, but rings a bell.

Yes he was alone is a hotel room, tripped on a rug and hit his head. He didn't call help and slowly bled to death.