If they're both consenting, then what's the problem? Assuming of course, they are both physically developed enough (gone through or in the process of puberty) to do it.
So, you all saying that the problem is that the younger person is too dumb? That's kind of a lousy excuse, after all, retarded people beyond a certain age can have sex.
Physical development unfortunately doesn't directly line up with mental development. If you really want to be able to bone 12-year-olds, there are plenty of third-world countries that would love to have you.
I believe 18 to be a much to high age limit, but the thing is, if laws didn't exist, then we would have 40 year old men who have charm, wit, a fatherly presence, taking advantage of 13 year old girls that are not mature enough to realize it.
Canada, however is 14 with consent from your parents or counselor. I believe the other person must also be within three years or something. I believe that this is a better system, since even though the US is 18, no one waits in worry of the legal issues