Well, if you want to use the historical model, then it made sense to marry and procreate young, since you might be dead at 30 with grandchildren, if not considerably before then. As to the age of consent for murder vs. consent for sex, murder is illegal but sometimes excusable under circumstances of self defence. It is also an act that is permanent for the victim and irreversable. We also have varying ages of consent for all kinds of adult activities, such as drinking, voting, driving etc. How on earth can you even advance an argument like that? The if there's grass on the field its game time model is absurd. Children are fully capable of crossing a busy intersection as soon as they can walk, but I doubt you advocate letting them do so. Just a thought.
regards