It was legal for 16 year olds to marry across the entire Anglosphere as recently as 1990, except for a couple of US states. Even by 2016, Australia was the only exception.

Did increasing the marriage age prevent teens from having sex? Nope. Average age at first intercourse for men and women remained pretty stable at 17-years-old across the Anglosphere.

However, this policy, combined with widespread use of contraception and abortion, did reduce teenage pregnancy by 70-80% from 1990 to 2026. At the same time, births out of wedlock have increased from ~25% to ~45%.

If you want to increase TFR and create stable families, raising the age of marriage is clearly counterproductive. People should be encouraged to marry their high school sweethearts and have their first kid around 20 years old.