without additional restrictions and fees...
IMO the physical ability to do work, plus a H.S. education (roughly 60% of U.S. population) are the requirements for those "Nestlings" to fly off on their own at 18 yrs....btw, there are clearly millions of citizens who will NEVER achieve "Adulthood", so factor that in.
As for the AR-15s...NO ONE should be able to purchase them...if folks want to shoot them, a) join the military, or b) find a range that lets you use "Their" weapons....