My theory why Ash is still 10 in BW... I think after the horrible loss against Tobias he has fallen into a dream, between the struggle of fulfilling his dream and the countless mistakes, which carried into Unova. The final cake was Zekrom after Pikachu lost his electric powers for a while. And after the Decolora Islands he will finally wake up and realise it was only a bad dream (the trailer for the last episodes makes Iris and Cilian disappear as pictures towards the sun) and swears he will train really for his dream as a Pokémon master.
His Lapras was still young while in the Orange Islands, but in his cameo in Johto he is now leader of the Lapras group and also a bit older and wiser.
I believe there is aging in the Pokémon world, but it doesn`t appear to the main characters really, especially Ash - losing a league and going to a new region as a rookie (which should be impossible because of his experience) - it calls a time loop, which he can`t break (and isn`t aware of it).
This is my thought of "Not Aging" - Ash is stuck into a dream caused by Tobias` Darkrai or is stuck in a time loop.