My personal opinion on the matter is that no one under the age of say 15 or 16 should be considered old enough to consent to sex. You can educate children about the protections against pregnancy and the consequences of their actions but that does not mean they are responsible enough or mature enough to truly understand what they may be getting themselves into.
For example, my nephew is going to be 7 years old in a few days and he is, and I am not be biased here, is probably a genius. He can read something and remember facts, tell you the scientific names of all the bugs he reads about, and reads at about a 4th or 5th grade level. He knows about the birds and the bees and hwere babies come from but that doesn't mean he is capable of making a well informed, rational, mature decision. It is my belief that untol the age where they are able to reason, to weight consequences are they truly able to make the right choices and able to deal with the choices they make if they happen to be the wrong ones.
That's my opinion on the subject. As for underage Dominants... well I don't think they really know what they want or have the dicipline to control thenmselves and not take it too far even if they do know what it is they want.