He's been around since Superman 13 in 1941, which makes him at the very youngest, as old as Dick Grayson currently is in the 20 to 30 range. Unlike Dick Grayson however, whose age when first becoming Robin is very well documented, I've never been sure how old Jimmy Olsen was supposed to be at the start of Superman's career. Was he a pre-teen? A teenager? A young adult?
Action Comics 865 claimed he was 22, which seems a bit young for a character who was working at the Daily Planet when Superman first appeared in Metropolis.