When the girl is pregnant probably !
and maybe when they want it early :)
There are many causes affecting human behavior. There is a different reason for every individual. However, there are reasons that tend to happen in most cases.

Early marriages could be caused by family problems. Sometimes, there could also be peer pressure. Early marriages can also be caused by early pregnancy, which also roots with my first two indicated reasons. Usually, teenagers become very curious and they tend to look after their curiosity.

The bad effects of early marriages come by when the young couple is not yet mentally ready. Moreover, some young couples that had children have not reached the legal age to work so they have to find other ways to cover their expenses. Still, it depends on how the couple will cope with their marriage. 

In my opinion, we can't really judge a couple's ability when married. No matter what age, it may work and also may not work. No one can tell.