In my family's culture (Russian Protestant), getting married early is not only common, but expected. My wife and I got married when I was 24, which was almost shocking in a community where the average was around 20. I've had many awkward conversation (of the "I will pray for you" type) with my grandmother, who sincerely believed I was making a huge mistake by choosing school instead of marriage.

Personally, I feel like I got married at the perfect time in my life (I was done with school, had a great job lined up, etc), and have never regretted that decision. My wife helped me re-focus on the important things, and inspired me to pursue my dream of starting a company.

However, the majority of my Russian friends who were married in their very early 20s have seemingly stagnated in their professional (and even personal) pursuits. Most were married either before or during college, and have either dropped out or are pursuing their education on a very limited part-time basis.

My experience is completely anecdotal, but it seems like getting married early (18-21) has significant drawbacks. Perhaps the sweet spot is somewhere between there and the current late-20s average.