For me, and he wasn't young when he died at 61, but when The Professor Died I wept, I mean I really wept. I'd seen Rush over 30 times, he was probably the best drummer I've ever seen live (yes I saw Moon, Bonham and many other great stickmen) and his lyric writing was amazing.
Mark Hollis (Talk Talk). He doesn't really meet the age criteria as he died at 64. I had in mind that he'd return to produce further work, and now know that he won't.