What does it mean when in the bible it says in the 13th year what is the 13th year?

You are likely referring to the age of puberty, when the boys went through ceremonies of various kinds to become men. Almost all boys married a few years after. If they did not marry by age 18, then they were considered extremely strange.

The early age of marriage is explained by the fact that the average people did not live past their 40s. The early marriage of course brought forth children who could be raised and married by the time their parents passed away.