Mechanics-wise, younger players will always have the upper hand. Reaction time means a lot in fast-paced games such as DotA, and older players just can't keep up.

As for in-game decision-making, experience is everything, and age means nothing. I have a friend who's only 15 this year, but he's been playing with me since he was 6. Technically, we share an equal amount of non-competitive experience, which makes him the better player if he chooses to move up to compies.