I didn't realize the United States military was so close to collapse that they have to draft Trekkies
...those that do will only be the best and brightest (once they have reached an age appropriate for their species, for humans this seems to be 17/18).
Actually, it seems that you only need to be bright if you are under
that age, like Wes in "Coming of Age". The other cadets we have seen haven't appeared particularly wunderkindly...