The all-time late bloomer for me is Oliver Parkinson. Pitched in the minors from 1968-1985, with over 200 wins. I decided to give him a shot at the bigs with my 1986 expansion Indianapolis Arrows. He made the rotation, and hasn't looked back. No editing on my part, either. I figure he's gotta slow down sometime, but not so far at age 40. A Nolan Ryan-esque freak of nature? Maybe being on a really good team keeps guys young?
I've tried the same thing with other long-time minor leaguers, but none of them have lasted more than a season or two as reserves.