Seems like it would benefit the politicians most. Teenagers are probably much easier to manipulate than older demographics... supposing you could get them to actually vote in the first place (since they're probably more apathetic than older demographics too), it'd be a nice percentage to control.
But I don't know why anyone would vote for this... I mean, if a whole new section of the population is allowed to vote, that suddenly makes my vote worth less! And don't try to compare this to women's suffrage; women don't turn into men in 4 years. Usually.