Why so young? a person hardly knows who they are at 25, and even less so at 20. Hard life lessons and crises of values hit most people around the age of 28-29, when the planet Saturn, the teacher, returns to the place of birth after completing its first full orbit around the sun. Before that, most people are just living out their parents' values and the expectations inculcated from their social environment and education.
From my life experience, it is wiser to wait until early 30's to marry--still young enough to have children, old enough to have had some life experience and self-discovery, and hopefully on a path for career and financial stability to sustain the partnership without constant anxiety and bickering over finances.
Are there advantages to marrying young? I don't see any.