Jared: I disagree with having a set age requirement for any activity, whether drinking, driving, or having sex. I know many 9 year olds who could kick your ass at video games requiring amazing hand-eye coordination to master–a level of skill that, depending on your age, you may be physically incapable of achieving. Are all 9 year olds capable of driving cars safely? No, but many are, in fact, I wager there are more 9 year olds who can safely drive cars then there are 70 year olds, yet the latter is allowed to drive freely while the former is not. People should be allowed to drive cars if they have the required skills, demonstrated by an appropriate driving test (and I would agree today’s tests are not stringent enough, letting far too many people through who are poor drivers)–whether they are 9 years old or 90 year olds.
Sure, you can hurt yourself when having sex, and you can hurt others, but that's true whether you're a child or adult. You say not many 6 year olds understand the dangers of AIDS and birth control, but I know dozens of adults who either don't understand the dangers of STDs and unwanted pregnancy or choose to ignore them. But we allow adults to engage in risky sexual behavior, because it only harms the parties who voluntarily consent to it--and such is the very reason we should allow it for children. Yes, some children would make poor decisions (though not as many as adults--most children aren't interested in sex at a young age), but that's life. People are stupid ignorant fucks who are born to lead a life of suffering and die in misery. That's life.
I would say you can't perform sexual acts on a baby unless it consents to them--and since generally, the only way you know consent is through language, you're unlikely to receive consent from a baby (though if the baby pulls down your pants and sticks your dick in its mouth, then I would consider that a form of consent as well).
Your argument about having an age of consent is at its root about protecting people from the consequences of their own decisions. Obviously, I disagree. You fuck up, you live with the consequences of your poor choices--that's my philosophy, and I apply it equally to children and adults. Will some people make choices they later regret under this philosophy? Sure. But they will at least be able to make those choices for themselves, without society first preapproving their 'fitness' to make those choices.
I do think sexuality ranges on a continuum, from complete homosexuality (viewing members of one's opposite sex as one views a rock or a tree--elliciting no sexual response whatsoever) to complete hetrosexuality (the converse). Very few people fall on the extremes, I think.
Michelle: I don't know if your 6 year old neice is capable of understanding all the risks and dangers of sexual conduct. Probably not, but I know dozens of adults who, even if they are capable of understanding those dangers, simply don't--or, in even more cases, who understand the dangers and simply don't give a fuck. Am I going to deny these people the right of harming their own bodies? Nope. As long as they don't harm me, they can do whatever the fuck they want, be they 6 or 18. They'll make decisions they regret, sure, but who doesn't. That's life.
Diesel: Go ahead and tell me you've never slept with a woman who had sex with you SOLELY because she wanted attention. It happens all the time. People sleep with others because they want attention. So what? Let people do what they want with their own bodies, stop trying to determine if they are 'fit'--there is no objective standard of fitness available, and everyone you contribe will be the result of your own subjective view of the world.
As for allowing children to vote, I think that would result in no shitter a society than our own, where the stupid, the poor, and the imbecile vote together with the educated and intelligent. In an ideal society, there would be no voting, because you would pay directly for the services you require, whether child or adult.
Anonymous: I'm sure sexual abuse does lead to psychological problems. If someone stuck his dick in your ass (or pussy) without your consent, it might lead to psychological problems in you, too. But anthropology and psychology will tell you that consenting sexual activity doesn't lead to psychological problems in a society that doesn't stigmatize that sexual activity--whether for children or adults. The sexual practices involving children in other societies (existing or time past) would no doubt shock you, but the societies functioned without a hitch. Your Judeo-Christian worldview is cramping your objectivity.