There's actually quite a bit of interesting philosophy that intersects with this question.
At the same time, as a couple of the comments on the question suggest, things can easily go off the rails (in terms of being on-topic) if this becomes a free-ranging discussion of why people stop teenagers from having sex and whether or not teenagers should have sex. So I'm going to restrict my answer to this question:
What are the general reasons that human sexuality is treated differently than the sexuality of other animals in terms of expectations about how old they are before they have babies? What are some of the frameworks behind the reasons people propose (across many cultures) for controlling this?
A major axis here is ethical naturalism and the naturalistic fallacy.
Ethical naturalism is ideas like:
- Aristotle's virtue ethic that builds on the human function (Nicomachean Ethics Book I)
- Natural law (at least it in its classical formulations).
The naturalistic fallacy is a mirror image of this in the claim that it is a mistake to think something is good just because it is "natural" (where nature is a word subject to many meanings).
In support of the former, if we don't eat, we die. Therefore not eating is bad.
In support of the latter, most animals the size of humans are willing to rip others apart and use them for food.
Your question in turn asks "why shouldn't humans reproduce like animals as soon as they reach bodily maturity?"
Those answering that they shouldn't would build their answer on either :
- a claim that it's an instance of the naturalistic fallacy to imagine we should echo animals in this domain
- a claim that the analogy is inept.
The more interesting route is 2. Humans differ from most animals if not in terms of rationality itself but in terms of the depth with which they engage in reasoning and the degree to which culture informs their actions.
For starters, we don't reach adult maturity until a much later age than many animals. Further, our children are more fragile compared to the children of most other animals for longer. Third, due to the way in which we develop, we require a lot more of the parents than many other animals.