To add a few comments from a personal angle, I was 15 I had never crushed on any boy my own age in real life. I had aesthetic/admiration crushes on musicians and actors significantly older than myself, but I didn't fantasize about dating them. Instead I based fictional characters on them and wrote romances for them with female characters I'd made up (who were nothing like me in either appearance or personality, but represented my idea of who would be an ideal match for them). Or, if they were already paired up fictionally with someone, I just wrote fanfic (Remington Steele & Laura Holt 4 EVA!).
I had my first actual real-life crush at 16, on a 19-year-old guy I'd met at camp one summer, and it lasted for nearly three years -- even though he lived hundreds of miles away, had never treated me as anything more than a friend, and we only saw each other once more during that time. I didn't even look twice at any other boys* until I'd convinced myself (finally) that nothing was going to come of it and I had to move on.
After that I had only two more crushes, one after the other, both lasting 2-3 years and never going beyond good friendship. I didn't date during that time, because the only guys who asked me out were so tentative about it I didn't even realize what they were asking until much later (but that was OK because I wasn't interested in them anyway). My friends were all dating and one of them went through about eight boyfriends during our teens and early twenties, but even though I longed for romance and marriage, I had yet to meet a guy I liked who was equally interested in me.
(To finish the story, by the time I connected with the man who would become my husband, I was 27. He was totally worth the wait, but it was a long wait. However, the best part about it was that he'd been through pretty much exactly the same process as I had, so I was the first person he'd ever dated as well. I don't think anybody would ever have guessed that by looking at him - I certainly didn't.)
So every teen is different, and the way they approach/handle any romantic feelings they may have is different as well. Some fall in and out of infatuation on a regular basis; some go back and forth between a couple of longer-term interests; some find a single real-life attraction too absorbing and overwhelming to even think of entertaining another one at the same time (raises hand); and some just aren't ready or interested to think about that stuff at all yet.
And vivalalauren, I am revising that book right now.
* My newly minted crush on Alan Rickman didn't count. He just went into the stories instead.