Ash can't still be ten, in the ep where he returns to viridian city (back in the first or second season) he says something to the effect of "I can't believe it's already been a whole year since I was last here" ....or something like that, I think he also mentions in the second movie that it's been two or three years since he first started his pokemon quest, although I'm not totally sure about that last one.
but yeah, I'm pretty sure the game characters are around ten or so, when you check a pokemons height in the newer games, it says the main character is about 4'8, I don't think they could be that short if they were older than ten XD ....Although I still have a hard time believing Dawn's only ten, what kind of ten year old would run around the country side in a skirt that barely goes past her undies!? ...better yet what kind of parent would let thier child wear such a thing?