Spoiled...
...I wonder how much thought you've really given this?
The concept of expansion of individual rights seems to be causing you to be concerned about mis-use, about crime.
I think it was you who recently were interested in the drinking age, making the analogy that you can fight and die for your nation at 18 but can't drink a beer until 21.
Well, drinking at 18 is about what? Responsibility. When is an individual responsible enough to be able to drink, an activity that, by definition, alters your mental state with a poison?
When are they responsible enough to operate the device that is the #1 killer of people under 21, an automobile?
Voting?
All of these have numbers.
So, I ask you, when is an individual old enough to choose to defend themselves? Or own a handgun for sport? Or carry one into a bad part of town?
How old?