If I remember correctly, Gengis Khan’s wife was 5 or 6 years older. It makes sense. Young men are at the peak of their strength and energy for hunting and fighting. Women their age are not fully mature for childbearing in what was a very hard life for women as well. So it would make sense to have an older more physically mature woman for a mate. If Neanderthals were men at age 13, that would have been a bit young for the childbearing female.
According to wiki, his first wife was one year older. The marriage was arranged by their parents when she was 10 and he was 9, and they married 7 years later when he was about 16 and she was 17.
https://en.wikipedia.org/wiki/Genghis_Khan#B%C3%B6rte
It made sense for the son of a ruler to start making heirs as soon as he physically could (about 12 or 13) and it also made sense for his wife to be grown enough to safely bear children (17 or older)