Technically speaking females are fertile before that. Then, humans are much more fragile compared to other species after birth and require extra care. This is b

Technically speaking females are fertile before that. Then, humans are much more fragile compared to other species after birth and require extra care. This is because as the head volume increased, they had to be born ahead of time otherwise they couldn't make it through the "birth canal". Now, anatomically modern humans do not differ much from humans over 10,000 years ago. Parenting back then was basically bring food and keep predators away, not studying and getting higher education and getting a job to provide to your family. Modern lifestyle doesn't have to do much with what is good biologically: oocyte quality peaks on early 20s and decreases with age.