One might take the road that 18 years are not responsible. Evidence shows not full developed front cortices that inhibit risky behavior. That we let them have guns in the military is ok because they are tightly supervised.
That is the tack taken by one of our 'members' in the school/ccw thread. Youth cannot be trusted.
However, the reasonable counterpoint is why do we let the irrational vote?
Look who we have elected recently from all the parties!
Seriously, the argument will be reasonable restrictions and wild kids need to be restricted. See if that works as it would have to define voting rights as less risky than gun rights.