[Totally from my life experience]
In Bangladesh for low-income people, girls get married at an early age. It is for 13 to 18 years old girls. It is not like the guardians always forced them, they wanted it too with some exceptions in both villages and cities.
I have asked one village person why they do that and why they don't make their girls study more. They replied, there are some reasons, like if the girl study more then they have to find a man who had got degree more than that girl has. Men don't want to marry a woman who has more degrees than him. And in the village, it is difficult to find a very well-educated man. And when any male became more educated, they demand more money from the girl's parents to get married. Taking money from the girl's parents is becoming a tradition in Bangladesh. Without asking, the boy's parents expect many things like full house furniture from the girl's parents.
Another reason is when a girl grows up she started to get disturbed by men on her way to school or anywhere else. Many men in Bangladesh have a mindset that, if a girl is outside then they can disturb her the way they want and get pleasure from that. To save a girl from this situation, parents get them married as soon as possible.
In the city, I asked a parent, why she is getting her daughter married at an early age. They said, she started falling in love with men. The parent doesn't know when she will become pregnant before marrying, so they want her to get married as soon as possible.