For me personally, I just think it's because you're tied down to someone and when you're young that's your time to do everything and explore the world and stuff and marriage, while it doesn't necessarily end that, it usually does if you have a normal marriage. I just think that's why some people think it's boring. I wouldn't know, I've never been married.
You can explore, but with two tickets instead of one. You can do things, but with your hand in your loves'. Right? Yeah, you have to make some compromises but that's life...
But that's exactly the point. When you're young after having been tied down to school or your parents or whoever for most of your life, marriage would just be another tie. And ties aren't necessarily a bad thing, I just think a lot of people want to wait until they've experienced more until they get married and settle down (hence the term, "settle down"). I mean, if you become a couple who travels and all that, then good. That sounds like a pretty awesome relationship. But most couples aren't like that. Most couples get married, move to the suburbs or somewhere safe and such, have a couple of kids, and spend the rest of their life together. It's not bad, but I think people just want to do things before settling down like that (especially if you have kids).
My cousin (who got married at 23) said that married life isn't necessarily a bore if you truly love each other and can find ways to keep being interested in the relationship.
I'd love to get married young