@Dissident Sound @Condor I would surmise that the reason age is used as the primary criteria for consent is based on brain development and the legality of other rights one assumes at 18. It might be arbitrary but rather than create a system of IQ tests and evaluation (trafficking, brainwashing, gaslighting the victim) it is easier to go by an age whether the person is ready by then or not. 

 The fact is this. Government should stay out of those things and it should all be regulated by common sense and strong parents. If a 30 yo comes near my underage daughter I do not need the government guideline. I'll fix it my way.