What if she willingly went to a bank and signed a contract to buy a house? Would it be legal? No. Why? Because we say that 14 year-olds aren't mature enough to make those decisions, willing or not.
Actually, that's because that's a CONTRACT, but a minor with possession of alcohol still gets in trouble as would a minor who pulls the trigger in a murder or helps after teh fact.
Minors' ability to consent is very limited but it does exist. It is not a fast rule in the way you are portraying it.