Well? What is it people? I swear if someone asks me why I made Ash 18 one more time I swear I'm gonna....
Anywhore. In the first Episode in the Pokemon series; Ash see Ho-Oh. According to a dex entry (From updated game of course B&W) "It possesses seven-colored wings. It is said that those who see Ho-Oh are promised an eternal happiness."
Ash had seen the Ho-Oh in the first episode. Which leads us back to the question "What is eternal Happiness for Ash?"
The answer is simple; being on his Pokemon Journey, therefore he is forever TEN.
To go with the story of Pokemon Black & Blue; Ash's eternal Happiness is no longer being on his Pokemon Journey, so therefore he ages.
Welp, that's all.