Personally, I think the number is arbitrary and there are 20 year olds incapable of making responsible decisions, and 15 year olds that are. But that's the world we live in, it's a compromise we make with our community to prevent abuse of little kids, and a way to set expectations of people, "by this age you need to be capable of looking out for yourself or you're going to have a hard time". I'm OK with this particular compromise, arbitrary though it may be. I don't know that there is a smarter alternative, and half your age plus seven is most definitely further from the goal than a set age when someone is expected to be able to navigate the world on their own. In older times (and in some less developed cultures even today), people were expected to be capable at puberty, in others it was when their fathers said they could, and we don't do that anymore because those systems almost always lead to fathers selling their children. So I'm happy with the current rule.