He is canonically at least 12.
He introduced himself as 11 in the start of DP, and it has been one year since then that he is in his BW journey.
Except at the beginning of Best Wishes they reintroduce him again as 10. Best Wishes, as enjoyable as it's been, has been wiping its ass with continuity all throughout both seasons. Ignoring that Pikachu, even without his electric attacks at the beginning, should have been strong enough to wipe out any starter Shooty chose with a Quick Attack or Iron Tail (which did not happen), there is still absolutely no excuse for that Koffing Pokedex fiasco.
I've just decided that in order to prevent hurting my brain, I will treat Best Wishes as a trolling AU series that keeps listing reasons it should not be considered canon then boldly saying to your face that it is canon because older characters keep popping back up.