Please don't bring up the design issue with May, it's already been brought up hundreds of times. I never noticed it until people mentioned it, and people who do notice it and talk about it all the time are perverted, in my opinion. She's 10, she's a cartoon character, and, well, people are different. Not all 10-year-olds look the same. Besides, studies have shown that every generation is taller than the last, so May probably developed faster, since she'll end up being taller than her mom is.
Anyways, about Ash. I'd say the characters in Pokemon are like Mickey Mouse and his friends. I mean, sure, we've seen Ash when he was a kid, and we've seen Mickey when he was a baby, but we've never seen them older than what we know them as, since they're ageless. The kids in Doraemon have never aged, and since Pokemon seems like it'll be forever popular and loved, like Doraemon, I don't see why the producers would mess with a good thing and let the characters age.