@Eiregoat @Vivernu @Ricotta Nature/God made them fertile at 12 for a reason

When conditions were bad menarches have been recorded at 16ish, when conditions are good (like the med thing Ricotta mentioned) then menarche happens at around 12. This is so they can have kids

What’s really interesting, though, is that this getting of periods when you’re sixteen is not the archetypal human state of nature either. It was kind of a blip that happened at the beginning of modern times—a retardation that followed a deterioration of living conditions even as the march of human progress achieved lots of impressive milestones.
https://www.lastwordonnothing.com/2018/01/02/the-historically-slippery-age-of-puberty/

The continued reduction in the age of onset of puberty should not be treated as a biological anomaly. It is likely that some 20 000 years ago, humans had already evolved to experience menarche at around 12 years and at present many countries are moving back to this position.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2465479/

Meds for example explicitly married girls off at 12 to an older man for reproduction
https://www.thecollector.com/pedophilia-ancient-greece-rome/