There are some very odd fan theories out there, including this one which suggests Pokémon's Ash is essentially immortal.
I mean, how else can you explain he always looks ten years old? Well, I suppose you could just explain it by the fact it's a cartoon and they rarely show the linear progression of time, but let's face it, that's not as exciting.
This theory suggests his encounter with the Ho-oh in the very first episode of Pokémon imbued him with the ability to live forever. Let's take a look at tyhe HeartGold and SoulSilver Pokedex entry of the Ho-oh to find out more.
Poké fan theorists took this to mean that Ash has gained eternal happiness, and what makes Ash happy? Well, being on a Pokémon journey of course. Ergo, Ash is always ten years old.
That's great. But it doesn't exactly explain how everybody else stays the same age too...