It is strange indeed, Pokemon anime gives some signs of time passing, like in one early phase (Ep 67 Showdown at the Poke-Corral) it was stated that Ash had to wait 2 months to Pokemon Kanto League starting (Ep 76 All Fired Up). Taking that into account, one season should correspond a certain amount of time. But still, Ash and other characters seem to stay about the same. In other animes like Dragonball or Naruto the characters age, but not in Pokemon...?