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.
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.