Getting married at a young age won't make you poor.Having kids at a young age will make you poor for sure.
I don't know about your country,but rising a kid from it's birth to the age of 18 costs more than 150,000 USD in the USA.The cost might be lower in the other countries,but it's still a significant amount of money.
But then if you have a rich family there is no need to worry about that. The problem is the crab mentality. They do not consider things and just make decisions in an instant. They tend to do things without even realizing what will be its effect for themselves and to the community and most importantly to the economy.
That is only if we have a rich family who will support in the financial section. But not all person can have this, and some of them still struggle to continue their lives. Some of the young age doesn't think about the future, and they got married because they feel that they love each other, but they don't prepare for the next things after the marriage. The lives after marriage are essential because the young age still has a big ego which sometimes could lead to divorce.