It's not about age as it is being truly prepared for that type of commitment as well as having one's house in order to give themselves the best chance of succeeding. Most marriages fail become people select the wrong mate for themselves. This often happens when one has not figured out who they them self are, what they want, or need in a mate. Essentially they have not experienced enough life or have done the necessary introspective thinking. Lack maturity & discipline. Still want to party like they are single.
Another leading cause for divorce is getting married for the "wrong reasons". This may vary from an unplanned pregnancy, an ultimatum was given, someone is being shipped off to war, "all my friends are getting married", it's an age goal, or What the hell? (might as well)...etc
Last but not least having no sound plan for the future often leads to divorce. People get married without having an education or training to establish a career path, start having children before they can afford to care for them, carrying large amounts of debt and hope their parents will rescue them or it will magically take care of itself. Stress on top of learning to share your life with someone is too much to bear for a lot of folks.