I haven't watched any Diamond and Pearl yet, so can someone enlighten me as to whether Ash is taller than Dawn?
Misty was taller than Ash; and Ash was like just barely taller than May. I think when Misty returned for those three episodes in Hoenn, her and Ash were about the same height.
There's a lot of different theories here but..mine is that Ash died once. The episode with the Pokemon Tower in Lavender Town, Ash died remember? He was brought back to life, and now he can't age. End of story. I think the PDD theory is kinda funny though haha.
Ash's apparent lack of aging is actually an easy explanation. If he aged, the kids who watch Pokemon wouldn't be able to relate to him. Ash's behavior throughout the series is consistent with that of a ten year old. He does one thing right, and gets extremely overconfident. He gets mad when he's not right, and runs off to be alone. He has a much, much bigger interest in Pokemon (and battling) than he does with girls. Several times throughout the series, a girl has had an attraction to Ash. He ignored them until they suggested a battle. (I can relate to stuff like that..back when I was ten, a girl would show interest in me, but I'd act indifferent. And then she'd be like, "Hey, let's see who can jump farthest off the swings!" and I'd be like "HELLS YEAH!")
Ash should be about 16 years old IMO. I just can't see travelling through 3 regions, the Orange Islands and the Battle Frontier, and Ash only being 10 or 11 years old. Then again, the realization dawns on me that I'm watching a cartoon. Ooooohhhh. Right.
Two of the easiest ways to judge age in anime (especially in males) is by height or a beard. Adults are (usually) much taller than our main characters. Most mid-life adults or seniors will have beards. I remember seeing artwork for D/P once, and Brock had a small beard. I was like, whoah! Brock actually aged. But alas, that was for naught as I caught part of a D/P episode on TV a few weeks ago and Brock had no beard. Oh well.
Age is also pretty obvious among women in anime. Look at (Misty, May, Dawn), Jessie, and Ash's mom. The age difference is apparent. Speaking of Jessie, there was a Johto episode where she's talking to James about capturing Pikachu (haha that's like....every episode.....). James mentions that he's tired of chasing "the twerps". Jessie replies that they've been chasing them for "years", and that they can't give up now.
In a nutshell, Ash should be around 16, but he's really around 10-12. He remains in that age range so kids who watch the show can relate to him and the challenges he faces.