I think there's a good reason for 18 being the right age. Children under 18 haven't fully developed, and may be more apt to make poor decisions based solely on emotion.
That and you are legally responsible for your own actions at 18. If you aren't legally responsible for your actions, I would think it wouldn't be right to carry a firearm.
Bear in mind that I am only talking about CC/OC of a pistol in public. On your own land, or while outdoors, I don't see any reason that anyone but the parent should decide when a child is old enough to carry and use a gun. I do, however, disagree with allowing children under 18 to carry guns at school. I think teachers and anyone over 18 should be allowed to be armed, but I don't think it's ok to let your 10 year old pack his glock to class. 10 year olds tend to over-react and get in fights. They tend to bully or (even worse in a gun situation) be bullied. They may not have a fully adult grasp on what the appropriate response to bullying is. Shooting isn't it. Threatening to shoot isn't either. Most people under 18 aren't fully aware of the laws either, and couldn't be held responsible if they broke any gun laws. Not that I support any gun laws per say, but I do support the enforcement oflaw in general.
That being said, I think that it is the parent's decision when a child gets to own a gun or use a gun, but as for carrying a gun, that effects society, so I don't know about parents making that call. Maybe 16-17 would be a good thing to test and see if it works, but I would absolutely support 18 yo carry.