Why doesn't Ash age?
It's the question every Pokemon fan asks, and lots are sick of, but I think I've finally got a valid answer that doesn't sound like something out of a poorly written fantasy book.
Throughout the generations, Ash's physical appearance has seemed to mature over time, and though unintentional, is also valid evidence for this theory: the Pokemon world simply has longer years. While many places in Pokemon are based of real places, it's still not exactly a parallel universe to ours. The Pokemon world probably orbits around a much larger star than our sun, therefore, taking a longer path to complete a year opposed to our approximate 365 days. In our world, Ash probably would be 16 or so, but in his world, he's still ten.