19 seems a bit high for those times. It's known that in Ancient Greece the typical marriage age for women was 15. That was the case even 200 years ago, at least in Eastern European countries, AFAIK. I doubt that ancient tribespeople would care about modern day standards if even the African tribes of today don't care, but the AI does care apparently. I think it tries to be politically correct and avoid outputting something that would be against the cultural norms of the modern times.
The second issue is the marriage itself. Were people living hunter-gatherer lifestyle largely monogamous? If you consider all the people that have ever lived and thousands of cultures, it's weird to include marriage as a category, as if it was something universal and inherent to human biology. It should be: "typical age women became mothers".