My girlfriend carries and she just turned 19. I don't see anything wrong with it. She also frequents the gun range with me and we also do IDPA shooting matches twice a month, so she is well versed in the safety and responsibility of carrying.
Just as driving a car entails safety "training" I believe that carrying and handling a firearm should be taken just as seriously. These are the exact things I will instill in my son as well. Safety and responsibility.