I say you can start training whenever. Younger would be better but you can't go back in time so start now.
Remember how they said Anakin Skywalker was to old to start training? Well look how that turned out for him, one of the most powerful jedi ever. Sure he ended up becoming evil and a cyborg but you can just skip those parts (if you want to).
The Reigning, Defending, Undisputed MMAFNPFFL Grand Master Champion of The Universe!!!