Sexually mature? Definitely not. Able to procreate? That depends on nutrition, probably not, only a few years later (15-17).

Sexual maturity means that an organism as reached the capability of to reproduce.

Looked around in the literature. Apparently consensus is that paleolithic hunter-gathers had menarche earlier than agricultural groups. Around the same age as the current averages in developed countries.

Having menarche and having the capability to bring a child to term are different. Pregnancies at early age have a higher rate of miscarriage. It seems there is a question of definition of sexual maturity - for me it is the age of reliably giving birth, for you the capability of getting pregnant. I am fine with either as long as it is clear what we use.

And nutrition depends on whether they have someone to feed them.