As others have said, everyone is very different.
However, I do think religion does sometimes have something I do with people marrying young. One of my closest friends is getting married in a month to a guy to a guy she knew 4 months before getting engaged. By the time they are married, it won't quite have been a year. He's a vicar. She's training to be a vicar, we haven't explicitly talked about it, but our mutual friend and I both suspect that old 'no sex before marriage' might well be a big reason for the haste.
Some marriages last, others don't. I don't think you have to be together 50 odd years for it to be considered a successful marriage. I also think these days, people my own age are not necessarily in the right position to be getting married. In their early 20s my parents had a mortgage, had been working full time for several years. My friends and are living back at home with our parents, in a lot of debt from uni, and having trouble finding entry level, low paying jobs.