Why are there age limits? why can't i marry a 12-year old?

Literally because of feminists.

No one in the entire world had a problem with this and it was common to marry off young 13-14 year old girls to older men all over Europe and even America until the fucking Suffragettes and later Feminists ruined that for everyone by making it illegal.

It's social programming and manipulation only, there is no scientific basis to restrict it to such an arbitrary age. All cries of "not fully developed" and rekt by the fact nature says''s they are physically capable of childbirth at 13 due to first period, and mentally, it goes according to individual. Most adult women are not mentally mature enough to "handle" sex today anyways it's a bullshit excuse used by American puritans and idiot fathers who want their daughters to remain pure until death.