About what "age" did our more primative ancestors start mating?

About 4 billion years ago.

How is there any way to know that? Chimps start at about 10.

Girls in hunter-gatherer societies probably did not reach puberty until 16 or 17. 

When they reach sexual maturity or puberty. That is the time when females start to menstruate, grow the mammary glands, grow pubic hair.  Males experience an increase in male hormones and their penis increase in size. Both genders start releasing pheromones to attract the opposite sex. The armpit and pubic hair traps the phermones to make them last longer. When the phermone is consumed by bacteria, body odor is produced. Children have no phermones and no body odor. Therefore we can tell a person is sexually mature when they have pubic and armpit hair. 

Different human populations reach puberty at different ages.  Africans tend to reach puberty sooner than Europeans and Asians. 

I believe girls can become fertile around seven. I was interested in sex at eleven. Possibly younger girls who became pregnant may not be able to survive childbirth, so they perhaps observed this and tried to avoid sex early.