I think you're reading too much into my statement, Duncan. I'm not saying we should unilaterally permit rampant sexual orgies among teenagers. I'm saying educate them, responsibly not with fairy tales, to insure they know the problems and responsibilities of their actions. And if they DO become active, make sure they have access to condoms and birth control, where necessary, to insure they violently thrust into the world of adulthood by becoming parents while they are still, by our culture's standards, children.
As for when children should be treated as adults, this can vary from person to person and from activity to activity. Obviously nature turns children into fully functioning adults, sexually, at puberty. Emotionally they may not be ready, but this is a product of culture more than nature. If they were taught from early childhood that they would become adults at a certain age they would be more emotionally able to handle it.
As for drugs and alcohol, obviously no one is adult enough to handle putting toxins into their system, but it has been shown that most people under the age of about 19 or 20 are unable to properly deal with the effects of alcohol, biologically speaking, than older people can. Younger children can be seriously damaged by even small amounts of alcohol in their systems. So our culture has decided that it's better to restrict alcohol to those who are old enough to metabolize it more readily, which we have defined legally as 21 years old.
As for war, well, children have been going to war almost as long as men have. Again, puberty seemed to be the point at which a boy became a man, with all the responsibilities that implied, including going to war. Our culture has assigned the age of 18 to determine if a man is able to be sent to war, but it is arbitrary at best. Some might be mature enough to handle it at a younger age, some might never be mature enough. (Aside, sort of: I was intrigued, and pleased, when watching "Master and Commander" by their somewhat historically accurate portrayal of what we would consider children as crew members of a ship of war. This was quite common throughout most of our history. Not surprisingly, though, they chose to ignore the sexual side of this in the movie.)
So yes, Duncan, sometimes we need to treat our children as little adults. Make them aware of the problems which can occur when engaging in sexual activities, emotional and physical. Teach them the reasons they are feeling what they are feeling, and how to control those feelings. Let them know that masturbation, far from being the dirty, filthy habit that some would procaim, is actually a healthy activity, albeit one which should be practiced in private and discretely (kinks aside, of course). Teach them that they should feel free to say no if they don't think they are ready, and that they shouldn't allow themselves to be pressured into sex. But sooner or later some kids are going to experiment. It's what kids do. It's how they learn how to be adults. Regardless of what morality you try to teach them, some kids are going to want to make their own rules. All you can do then is try to steer them in the way you think is right and hope they don't fuck up their lives while doing it.