Cort321 said: He looks older because his body was full of testosterone so his balls dropped faster... and became the Earth's center of gravity... yeah.
In the original manga I'm pretty sure he was 13 in the original Arc, 15-16 in the Arc where Yellow was the protagonist and 17+ by the time Gold was the protagonist.
The manga, which is the basis for most fan art, is a lot different from the games even though they share common story threads. In Pokemon Special, by now, I'm reasonably certain he's older than twenty.