The Pony field/league prepares kids for HS ball if you start there at 11 - 12 years old. You get a slightly bigger field, HS type rules and the kids learn alot. AAU programs use that size field and NFHS rules and there is a much smaller learning curve when they turn 13 and hit the big diamond. Most of these players are better prepared than the kids coming from LL.
I think once you get to be 13 then you should just get on the full size diamond. If there's a choice of Pony or Babe Ruth and you're 13, then go with BR. I guess if the kid is tiny and weak or not very baseball savvy, then maybe Pony would be ok, but they're gonna have to make the adjustment anyway, so why not just do it once and be done with it.