We're not just talking about responsibility here. We're talking about young people getting hurt. People talk about sex education as if its all about contraception and what parts go where. It should also be about knowing yourself well enough to know what you want. It should be about how to say "no" to someone who's pressuring you into more than you want to give. It should be about caring and sharing and boundaries. But its not. And I know a lot of women who were pressured into things they were not ready for. These pressures are often very subtle, but to an insecure young woman they are very powerful.
I understand that the age is arbitrary. But the human brain simply doesn't mature very fast. I tried to find the info but I haven't found it yet. I seem to recall that one of the important parts of the brain re: decision making isn't mature yet, even in the late teens.
I believe it would be possible to teach our children well enough that we might not need so many legal controls on what they can do sexually. I believe we are nowhere near that level of maturity as a species. You throw in religious nut jobs who think its ok to prohibit sex education at all, and you have a recipe for disaster. As I understand it, the more a person is educated about sex, about options, about boundaries ... the longer they tend to wait, and the more responsible they are about contraception and being kind to one another. That's certainly been the case in my extended family. The right wing christian young-earth creationist nutjobs have tons of teen pregnancies in their families. The "men" aren't anywhere to be found, and these children are raising their babies with their parent's help. Education is disrupted, and often not re-started. Their lives and potentials are forever stunted. I find it very, very sad.