And yet your position in post 38 is applicable to age 16 in states where they are assumed to be sufficiently responsible for their actions to enter a marriage contract. The mentioned distinction of the age of voting is arbitrary and emotionally based if other state qualifiers of personal responsibility are not valid.
In what states does one need special permission from the government in order to vote at age 18? Obtaining a marriage license or gaining independence at age 16 is normally granted on a case by case basis, but any 18-year old may register to vote, providing they have not done anything to have their civil rights restricted (in which case they couldn't possess firearms anyway, no matter what their age).
The topic of marriage is not addressed in the Bill of Rights because it is correctly viewed as a matter for the states to decide. The right to keep and bear arms made it into the Bill of Rights precisely because it is a right of all citizens. If a state wants to allow any 16-year old to marry, no questions asked, what does that have to do with the Bill of Rights?
Also, under most circumstances, offenses committed by juveniles (i.e., under 18) are not part of their permanent record. Normally, except for capital offenses, juveniles are charged, tried and sentenced as juveniles. But it's the exceptions that prove the rule: At 18, citizens are responsible for their actions. If they are accountable as adults, their rights as adults should not be denied.
Finally, to your point. If a state were to decide that full rights of citizenship could begin at 16, there may not be anything in federal law to stop them. I don't know one way or the other because I'm not a lawyer and have no interest in the research necessary to figure it out right now. However, I would tend toward restricting 2nd Amendment rights to citizens aged 18 and above. It's instructive that the Militia Act of 1792 called for participation beginning at age 18.
I hope it is now plain why I did not want to play this game. The Militia Act of 1792 restricted membership to white males aged 18 to 45, but I do not support denying the right to keep and bear arms to women, other races or those older than 45. If I don't point that out clearly, someone will accuse me of bigotry. The Act is just a guide to when citizens should be ready defend themselves and their communities. Similarly, the right to vote at one time belonged only to male property owners. That does not mean I would deny the right to keep and bear arms to anyone not a male property owner. (Again, I must be clear lest someone accuse me falsely.) I use the voting age as a guide because voting is the most fundamental rite of citizenship, and because 18 is the most accepted age for holding adults accountable for their actions.