Why wait until S13? If you're gonna slow build, a Pee Wee team that's gonna be looking for players this early is probably not a great fit. They are going to want to be more competitive and spend SP.
My suggestion is go ahead and build him now. Build him and get him on a maximum XP team (
http://goallineblitz.com/game/team.pl?team_id=7879). You'll be a S12 player that will always be levels ahead of the countless S12/D31 guys that are soon to come.
PM me. I've got several positions that I can offer max XP/Playing time to.