I think there are a few reasons for this. Perhaps the most obvious reason is that our modern civilization gets married later in life than most cultures throughout the history of the world. In many cultures before our day, girls got married shortly after puberty, and guys would be husbands by the age of 18. Obviously times were different then, so I wouldn't recommend that a college freshman propose to an eighth grade girl.
One reason we marry so late is because our culture tends to mature a bit late. In so many other societies around the globe, boys become men as teens. They get their jobs, they learn to begin to take care of themselves, and they prepare to carry the responsibilities of a family. In America, we've got 28-year-old men who live in their mom's basement and play video games until one in the morning. Granted, our civilization has people who do mature early, but our society has more advanced learning as well, and many people choose to pursue education or career goals before family life. That's our choice.
The reason I get into this is to show that God didn't make a mistake when he made us. We've just changed the more traditional marriage age on our own. Again, I would not suggest that you get married early, but that you be wise in the desires you stir up in your body.
The Song of Songs 3:5 says, "Do not stir up, do not arouse love before its own time." Unfortunately, the media stirs up the desires for us. I've seen studies that showed that the average teen sees 14,000 sexual references and innuendos on television each year!
Now, you may be thinking, "I'm not stirring up anything. These desires are there anyway!" Know that God himself has given you these sexual desires. Remember in the book of Genesis when Adam saw Eve for the first time? He gasped, "This one, at last, is flesh of my flesh, and bone of my bones." He saw in her some completion of himself, and as a response, he felt drawn to make a gift of himself to her. This desire to make a gift of yourself to a woman is stamped into your body, and into her body. It is good!
But we also have these sexual desires as singles so that we can learn to control them. Unless a man learns sexual self-control before marriage, he will be incapable of loving a woman inside of marriage. He'll just use her. To control your desires now is to train yourself in faithfulness. After all, if you give in to the desires now, you'll find that you are not fulfilled. One young man said, "I began to notice that the more sex I had, the more I wanted. I had always heard that having sex was a way to get rid of sexual tension, but the opposite was true. Having sex increased my desire. It was like a drug. I couldn't stop myself, yet at the same time, I wasn't satisfied at all."
So, the solution to our desires as singles is not to give in to them, but to master them for the sake of love. In my own relationships, I have found that the ones that were easiest to keep chaste were the ones where we stayed the purest. Modern logic would tell you otherwise--that if you want to be abstinent, you need to let out your sexual libido somehow (by doing other sexual "stuff") and that will keep you from going all the way. Every man who is honest with himself will realize what a joke this is. You just desire it all the more.
What often makes this more challenging for teens is that sometimes it seems like everyone in high school is in an intense dating relationship except for you. In reality, the majority is not, but all we see are those who seem to have a date every weekend. In an effort to have the intimacy that we think others are receiving, we might jump into dating relationships, while we're still a decade away from marriage. We begin to get emotionally closer to the other, and our bodies desire the same closeness. One solution to this is to look into the concept of courtship.