Two answers. One, it's your car. Two, it's your kid. The answer should be obvious.
You avoided answering the question.
Who's decision is it? Who decides if someone is capable of giving consent?
If my 10 year old daughter feels that she is old enough to have sex, but I as her parent disagree, who has the final say? Or if she feels that she is old enough to drink, but I disagree, who makes the call?
Kids think they are capable of doing all sorts of shit that they really are not. As a society we have put age restrictions on certain things like drinking, driving, smoking, voting, etc. The age restrictions may seem arbitrary, but there is some sound thinking behind it like brain development, education, maturity, etc. Of course there are some kids that are more advanced than others, and not every child fits in to the same box, but in general the age of consent is a decent guidline that was arrived at not by accident. If you leave "consent" up to children that are not emotionally mature enough to engage in certain types of behaviour, there will be consequences for that child down the road. We have observed this in children over time, and therefor have put age restrictions on certain behavior. It's common sense.