There is no inconsistancy! Depending on the level of incapacity, mentally challenged persons are routinely denied the ability to drive automobiles, purchase or possess firearms, enter into contracts, and many other things that 'normal' people are not denied, because of their lack of ability to make an "informed consent" to the ramifications and responsibilities that attenuate those rights and priviliges. The same concept applies to 'minors', as they too lack the mental ability to make an informed consent, because they cannot comprehend the ramifications and responsibilities that accompany those rights and priviliges.