It will take a hit but certainly won't disappear. Success in Dota 2 nowadays is not based in having fast reaction times and dexterity but in strategic planning and team cohesion. If top teams have a young player, it is usually in position 1 or 2, and many older players are also currently succeeding at those roles.

Besides, this regulation does not appear ban things such as a Dota club at school (online play would be forbidden, but local lobbies would not).