2. Less relationship experience.
I didn't marry my first-and-only boyfriend, but I know plenty of young wives who did. Now I know some young couples don't want to hear this, but it's easier to spot an unhealthy relationship when you've been through one (or a few). And no one wants to be the Test Girl for a guy who has no idea how to be in an adult relationship. (Get those mistakes out with past girlfriends, thank you.) Relationships are learning experiences and less experience means less learning. That's not to say you can't learn together—you absolutely can—but it can be a challenge.