A lot of characters in other anime don't age; it's not really a big deal that Ash is still 10 imho. My only gripe is that I wish they never mentioned his age to begin with - better for it to be left unknown so fans can speculate and make it whatever they'd be satisfied with.
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☾ never lose sight of your wish ☽
Actually, I believe the answer is from the very first episode of Pokemon. For those of you that don't remember, while Pikachu and Ash where out on the Route, Ash saw a large peacock bird Pokemon, Ho-Oh, flew over Ash and over the rainbow, glittering. In Ho-Oh's Pokedex entry, it says as Ho-Oh flies on his seven colored wings, anyone that sees it is granted Eternal happiness. Now Ash saw it, so Ash is granted eternal happiness. And what is Ash's wish/eternal happiness? To be on a Pokemon journey. That is why he is always ten. But I'm sure that it is meant to target young viewers, but that is my theory.