I see/hear about marriages started at 30+ 40+ 50+ all the time that fail. I see people pivot careers and industries in the middle years of their life. People tastes change all the time as they get older. Let's not pretend that when your brain finishes developing you suddenly have life figured out/know exactly what you want

I generally agree that getting married before 24 is a pretty risky move and you have to have thought it through very carefully, but the argument that "you don't know what you want for the rest of your life" is not the reason why that is. It relates more to life experience/emotional capability/massive foresight. Marriage is more than just "wanting something for the rest of your life", it's a commitment, it's not just some eternal desire you may/may not have