The Fault in Our Stars
The antagonist in this story isn't a person; it's something far, far worse. All the lead kids in the story are stalked by a pretty insidious enemy: cancer. The cancer that lives inside of them doesn't let them cut loose, and it certainly doesn't let them live with the delusion that they'll be young forever. Instead, they're constantly struggling against the reality of their own mortality and trying to figure out how to live "normal" teenage existences despite it.