The reason is back in ancient/medievel times, after some of the prophets that lived to be 800 years old, many people died before the age of 40. Now, in that period, you have to learn a craft, raise a family, and be sure your sons and daughters are ready for the rest of their lives. Now considering that most people cannot reproduce until 12, that gives you 28 years (if you get pregnant at that age) to establish a career, raise a family, and prepare your children to be ready for their life.

The only possible way we could survive is by marriage at early ages. This caused the population to grow, and because of modern medicine, we started to live longer but our bodies did not adapt to that.

It is believed that cave men did not live past 18 often.

So in essence, we needed to reproduce early on. Also, the laws of marriage are man made, not by God. We decided that children should not have children and raise families. Which I agree with no less, we no longer have to have kids so young, so we can enjoy life more.

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