In many countries, it’s customary for girls to marry at a very young age, voluntarily or not. This practice is detrimental to the human rights of women, as I argued before.
In the developing world, more than one third of women aged 20 to 24 report that they were married or in a union by the age of 18. (source)
This practice is often legally entrenched:
In 50 countries, the minimum legal age of marriage is lower for females. (source)
However, it seems that the law can also work the other way:
(source, click image to enlarge)
Or perhaps the causation goes the other way: countries where customs are against early marriage also adopt laws stipulating a high minimum marriage age. In general, we shouldn’t be too optimistic about the power of legislation.