a 13 year old can physically perform almost every other task. In 3rd world countries, a 13 years old male is a full fledged labor, farmer, fisherman and all. Its us as a society who assumed that 13 years old is not emotionally mature, and because we treat them as kids, they act as kids. Otherwise there is no medical proof that a 13 years old is not as responsible as 18 years old.
And that is what I said. Its all economics that drive the need of our adulthood age limit. Those societies that live humble lifestyle (basic food and shelter), not only 13 years old kids get married but live a very happy life.