Castro is moving a bit faster than most big men I've seen. It's possible to go even faster than Castro, it's just rare, and requires some special circumstances (high starting skills, very rigorous training, high trainer level, etc.).
The best inside skills I've seen on a Canadian big man at the start of age 20 was IS=15, ID=13, RB=13. Most prospects just aren't going to develop that quickly, but it can be done.