It's not unheard of, but it's rare for a guy to come up at that age and perform well. The greats do it, but not many others. Orlando Cabrera hit .280/.325/.414 in his first substantial exposure to major league pitching at 23. But at 24 and 25 he hit: .254/.293/.403 .237/.279/.393 Before turning it up in his late 20's and becoming what I would assume people would call an above average major league shortstop. The Reds' version of Alex Gonzalez hit .200/.229/.319 at age 23. I think he more or less falls into that line of average SS nowadays, although on an inconsistent basis. Michael Barrett came up at a younger age, with initial success, but hit .214/.277/.288 at 23, and continued to struggle for a few years, before turning in a few solid well above average years. Baseballreference.com has Davey Johnson as the closest comparison to Cedeno at age 23. He hit .257/.298/.351 before turning in a really nice career. Ryne Sandberg hit .261/.316/.351 at 23. It's hard to find a lot of comparables, because most guys got either sporadic play at most at that age (or weren't even up) or the guys who became superstars were already showing signs. As I've said before, I have my doubts about Cedeno. The worst thing that happened to him was being added to the 40-man roster so early, thus forcing a rush to the majors. Now he's in limbo, having played behind some truly awful SS in recent years. If he had more options it would have been nice to ease him in at a later age, and then hopefully have him ready now, but with the comfort of an option to go back to AAA.