User blog comment:Thailog/May episodes details/@comment-4684185-20120428194836/@comment-180.190.135.191-20120429021317

Look at it this way:

Dick is human, so is Zatanna, Rocket and Batgirl. Hence, they look like they got older. PLUS he probably went through puberty. ^^

THEN Superboy and M'gaan are aliens so I'm assuming they don't age as fast.