I'm with Callaway, MarkingDawg, PI, etc.
The start of puberty is a poor reason for a minor to have sex. Their body may be ready to reproduce, but that does not mean they can emotionally handle it.
Historically, young people had sex and started families at a young age, but a lot of things were different then. People didn't live as long as they do now, and psychology wasn't as advanced. People didn't seem to care how these norms psychologically impacted children. At one point, age seven was considered "the age of reason" ffs. We know now that seven-year-olds are children and still developing. Society has progressed a lot since ancient times regarding protecting children.
Adults should be responsible enough not to expose young people to romantic/sexual experiences they are not ready for. It is bad enough that children get talked into sex before they are emotionally ready by their peers.