Conversely, you could say that half the adult population is not mature enough to vote. Or how about your logic applied to Women during the Women's Suffrage movement. They didn't work as much or were as educated back then. Maybe they shouldn't vote- oh wait, nevermind. They were disenfranchised.

What about African Americans in the 1700's? Again, on average less educated and "mature" as one might say due to being a disenfranchised population.

The funniest thing about your vote, is that in 2 years from 16, they somehow magically become mature enough...