It probably varies by state, but in Illinois at least, I think he would have to be 18 years old before he took the GED.
They have this rule because the GED test is very easy to pass, and if not for the age requirement, every halfway bright twelve-year-old could pass it and go straight to college. If that happened, whatever high school the kid was skipping would lose out on its Federal funding for that student. Schools get paid by the Feds on a per kid, per day basis. This is the same reason they care so much about attendance in general - it's about the money.