George III was 38 in 1776. The original George III in Hamilton was Brian D'Arcy James, who was 47 when the show opened at The Public. The subsequent Georges on Broadway and London were between 30 and 40. It would seem that the creators feel that 30's is a good age for their George. Casting an actor of 82 years old would be a rather major casting departure. I would argue that the song is written as a '60s Britpop ditty in a whimsical, youthful Cliff Richard mode and having a very old man singing it would be....weird. But so is an 82-year old Hamlet and I hear that he was terrific. I'm not saying he shouldn't do it, but to say that his age is irrelevant seems silly.