I think I just kinda figured out the real reason. So many say if Ash aged kids would stop watching but I doubt that is the case. Think about it from Game Freak's angle. Let's pretend Game Freak decides to have a gen where an older character is let's say champion. So for the sake of the theory let's use Misty. Now let's say the devs age her by 10 yrs. only to have the next games back in the regular timeline or before the events of Gen 1. The writers can not just use time skips willy nilly. While Game Freak has not really done anything too drastic. (Misty got a new style in HG/SS but she was still a gym leader but if she was promoted to Champion for some reason unless the anime decided to use an anime only champion (Which would be odd)