This is the pokemon universe we're talking about. I don't think kids like Red will ever age, they just get new clothes.
Well, the periods in the stories, if mentioned, are not so large. RGBY -> GSC: 3 years. BW -> BW2: 2 years. It's not so surprising that the characters don't look so different after such comparatively small times, especially with former games' good old pixel look. However, you are absolutely right about the anime. But the manga charas are definetely aging.