One of my close friends is a hopeless romantic. She says I'm cynical, but honestly, I have a hard time buying teenaged romances in which seventeen and eighteen-year-olds are portrayed as soulmates. The human brain continues to develop into a person's early twenties. Teenagers change a great deal from one year to the next, both in terms of intellect and emotional maturity, and I think you would be hard pressed to find an eighteen-year-old who's ready to consider marriage. Even living in Utah, where many people get married at a young age, I don't know a single person my age who plans on marrying in the near future.
I've been told I'm mature for an eighteen-year-old. I get along with adults, I have older friends, and my adolescence passed with relatively little drama. Most of my friends have boyfriends, and I would consider them above average when it comes to emotional maturity and intelligence (for the most part). But the reality is, teens don't really think about marriage all that much. It's far in the future. Lightyears away. We think about college and boys and parties and sex (even if we aren't having it), but an informal poll of PIKOF (People I Know On Facebook) shows that marriage is the last thing on our minds. Perhaps this is a recent change; after all, thirty years ago women didn't go to college nearly as often, so once out of high school they settled down and started families. But our culture has shifted in recent years and people are getting married later and later. Again, this may sound cynical, but (generally) I don't think kids my age have the necessary level of self-awareness and maturity to experience "true love." We're still growing, learning, and trying to figure out who we are.
When writing romance in YA, you have to be careful with how far you go. Most people enjoy a good love story, but keep in mind your characters' ages, and try to write from the perspective of a teenager rather than an adult.