ANDY'S ANSWER: In my opinion, any teenager who is still going through puberty and under the care of someone else, should not be having sex at all.
The risk of physical, emotional, not to mention life-changing (and probably not in a good way) consequences is way too high.
Sex is not a game sport. Someone who is already dealing with the turmoil and changes of puberty doesn't need the added pressures and stress of dealing with a sexual relationship. Letting your hormones do your thinking for you is a dangerous practice as they rarely have any judgement, let alone good judgement.
For some reason teens think that having sex turns them into adults. In reality, making responsible decisions for yourself and others is what really determines how mature you are.