I'm under the impression that the Pokémon world has a larger rotation around its sun than our world, and thusly, 10 years is longer for them than it is for us. I plan to use this argument in court once the FBI seizes my hard drive for this.
I'm positive that they're not actually humans, but very-humanlike pokemon that invented pokeballs. Ash simply hasn't gained enough experience to evolve into Teenager yet.
His female companions, however, have been leveling up more for some reason. Maybe he has an unusual evolution requirement condition and they don't.