it hard to say kid like it or not . There is more possibility kid under 7-9 can get abused without consent . But after that biologically humans come to purity so likeness to sexual act can come naturally . Older time most of girls in world legally got married or gave birth around age of 12 upwards , this is new society made woman should wait to get sexual intercourse until come to financial and educational independent . What you say is constitutional law but biologically women ready to have sex after purity .
Biologically, children in their teenage years still have a body that is developing! It took 21 years to achieve full maturity. Thats why it was saying you will get tall until you reach 21 years old. Haven't you seen a biology book that has some photographs of how men and women obtained sexual maturity? Wikipedia has some of this information that you should just check. Humans have a very long childhood that takes nearly two decades to complete their mental and physical growth. This adolescence and transitional age are still developing bodies, never considered fully grown or ready to mate. It is known fact and that is the teenage or under age never considered as full mature and has not knowing of actions their doing or rational thinking. It is a biologically proven fact; hence, constitutional law has an exception for underage children's special minor law, which is given in every country. Hence, these underage people have exceptions such as drinking age, voting age, working age, and age for having sex.
You are saying that girls who are underage can have sex and have children. Yes, I know that my grandmother and her mother married underage. They had kids until they reached 40 years old. During my childhood, I had uncles and aunts who had gray hair, and some were still in school.
As you say, you can bear offspring during your early teenage years. But as I said before, this is a transitional age that has developing bodies, both mentally and physically; hence, underage mothers are suffering huge complications that could end in death. That is a reason for changing the law to be correlated with biological facts.