Looking back at those days when my parents got married young, I can say that it is all about family planning.
There are 8 of us brothers and sisters. If not then I do believe it will be a small family of two like now as my wife and I didn't get married when young.
So, in the past, people got married young so that they can build a family of many kids. But now marrying young may not be the same idea as in the past. Some are hooked to marriage as they got pregnant while others are impulse.
Again there are some who got planning of having more children for future. So, it is due to some of these factors people are marrying at younger age. For those who are currently seeking for career tend to get married in a later stage of life.