Why does Christopher Lloyd look so much different in a 84 episode of Cheers than in Back to the Future (1985)?

I think the episode is called I'll Be Seeing You. It's where a famous painter (Christopher Lloyd) comes to the bar, and asks to paint a picture of Diane. It's at least a two-part episode if this helps anybody remember the episode. I'm just wondering though, why does he look so young in Cheers, and so old in Back to the Future if they were only a year apart? I mean he looks like hes 30 in Cheers, but he looks at least 60 in Back to the Future.