The way I see this is that one should be allowed to exercise their rights at the age they become 'independent' because if you believe that our natural rights are given at birth, you have to either conclude that we should be able to exercise all our rights from birth (not a good idea, imo) or those rights should be temporarily given to our caretakers (parents) for them to regulate as we mature. IE, if a child's dad thinks he should have a 22 at 12y/o then that's okay. He takes the responsibility for exercising the child's rights.
Now, since 18y/o is the age in which most people leave home, it seems to me that it is the age in which you should have your rights. Prohibition never works - and drinking before 21 is prohibition and buying an handgun from an FFL is (a smaller) prohibition as well.
Now, here's the real question - the 26th amendment happened in the early 1970's. What have our legislators been doing for the last ~40 years to have not thought through the logic of the above two paragraphs?