Whose definition of underage, in prison, mad or incapable? I hope you don't mean the states. Every person should have the means to self defense available to them.
Don’t get tetchy!
The exact definition, where you draw the line, is obviously a matter for the citizens of each individual country, but there are surely some common sense reasons for disarming certain people. You would not give someone in prison a gun for the very simple reason that he or she would probably use it to get out of prison! You would not allow someone who declared that they were actually “the man in the moon” to have a gun, or indeed any sharp object. I have a friend whose hands shake so much with palsy that they would be physically incapable of aiming a gun. They would be a menace to anyone near them. Then there is the matter of children. Some would say minors should be x years old before they have a gun, some would say y years old.
The point is that there is a proportion of the population in any society who will be forbidden (perhaps temporarily) from owning firearms. Its unrealistic to think 100% of any population will have guns. Actually it unrealistic to think 100% of any population will do anything in common. For goodness sake, not 100% of the population agree the world is round.