Frankly I feel ppl think of marriage as a huge burden, which is not true.
When you marry young then you've many advantages.
First, you've a partner with whom you would be able to share your good and bad days. I think it's in early 20's that we start facing stress and that's when you need a companion most.
Second, starting early helps you grow together. Emotionally and physically there's a better compatibility between the partners.
Third, I 've seen ppl having affairs for 8-9 years and then parting ways, had they been married chances of 'em parting would 've been lesser is what I feel.
Fourth, chances 're you might 've kids early if you marry early (I think one should get married at 22 but not have a child till 25-27). Having kids early is something that you would be thankful about in the later stage of your life. As
@MastanKhan says have them (Kids) while you can still enjoy playing with them.
But the biggest disadvantage of being a parent at young age is that you're not matured enough to handle a child. Speaking from my own experience it was all about "hit and trial" for me.
Fifth, the biological cycle!! I think this should be the biggest reason why ppl should get married early.
But like everything else marrying early also has its pros and cons.
This is my take on this topic, and the references 're from my life.