Ash Ketchum, Whoa

[George]

The bastard never ages.
This also applies to South Park, Family Guy and other animated shows characters.
Why would they do that? O_o.

[Eric]

now that you say it... he does seem younger 0_0

[George]

Yes!
After the first league, and the first area (the one with the cool looking pokemon) he says himself that a year passed.
But after he crosses 2 or 3 more areas, he looks even younger.
He should be like 15 now, taller and smarter.
Brock as well. At the beginning he looked like an adult, now he looks like a 10 year old.

[dgpl]

I think
I figured it out
...
Ash never hit puberty.

He hit
REVERSE PUBERTY!!

[George]

And here am I thinking only Justin Bieber has that condition.

[Kaio-sama]

He feeds on the blood of young Pokemon in order to remain immortal.

Comedic reasoning aside its just a stagnant of the show. They don't like change if they did they'd have dropped Ash and started over with another main character from one of the games. Its something known as Simpsons fever no matter how long a show runs for the characters will never get older.