Marraige age

Because 7 1/2 billion people just aren't enough, eh? And because three years of education is all a female needs, right? Just enough to read a recipe, eh?

/sarcasm/

Just because there are 7.5 billion people doesn't mean that we still don't need new babies.

Marriage age has a lot to do with cultural and environmental factors. In general, woman who marry later are better educated and more successful. And raising the age of marriage tends to decrease the birth rate. There are certainly conditions where getting married at the age of 13 or 14 makes a lot of sense (and in hunter gatherer societies this is fairly common).

We live in a world with education, technology, antibiotics, democracy... all of these factors favor a later marriage.

Marriage and fertilized eggs are not related at all. I think the question you meant to ask is, "What is the youngest age that girls should start having sex?"