Eli was an old man even 20 years ago apparently lol
It just looked funny, Walton Goggon's pulled it off of course called he actually is a young guy irl, but Goodman obviously is the age of his character irl so it just looked weird Billy being young back then while Eli was still an old man with just a wig on his head.
I dunno i feel like they could have done more to make Goodman look younger for the flashback episode other than just putting a wig on him.