Brian De Palma
Francis Ford Coppola
Stevie Wonder
Robert DeNiro
Al Pacino
Joni Mitchell
Bruce Springsteen
Meryl Streep
KISS (just kidding, they were always shit.)
Wouldn't nearly everyone, if not everyone, who is considered a legend fall into that category? I can't think of any legend, nor anyone at all, who did better work when they were senior citizens than when they were young to middle-age. Aging deteriorates the mind and body, and the older someone gets, the less likely they are to be in touch with / appeal to the most lucrative (younger) audience.
Maybe Scorsese. There's also a debate that I've heard people say about Spielberg having a better second half of his career. I disagree with that one though. Judi Dench had a late career rise and Maggie Smith has been pretty consistent.
Almost everyone's best work is toward the forefront of their career, there's some kind of explanation for why this is that is specific to creative types, but I can't remember what it is off hand.
In any case, most of the ones I thought of have already been mentioned, so I'll just add:
The Rolling Stones
James Cameron
Robert Rodriguez
EDIT: Charles Bukowski. His later books and writings were significantly different enough from his early work, it seemed like the work of a different writer, and a bad one at that.