That linked article says 0.5% of adults and 1.4% of teenagers are transgender. Maybe you misread the stat that says 18% of transgender people are teenagers, com

That linked article says 0.5% of adults and 1.4% of teenagers are transgender. Maybe you misread the stat that says 18% of transgender people are teenagers, compared to 7.5% of the general population being teen-aged. I think there's some truth to the idea that people who might have previously "just" been gender non-conforming now identify as transgender.