The Internet is mainly the reason why young Americans are transitioning. Transgender wasn't a known thing back then. It was still considered very foreign and weird. Any person who might have been transgender didn't know it themselves, and probably identified with other, more known labels like drag queens, gay or lesbian. Because of the Internet, the concept of transsexuality became more known and people started realizing they identified with the label more.
Additionally, as some have said, times are different now and society as a whole has become more accepting of trans people. This has given many young Americans the courage to transition, whereas young Americans in the past were more afraid to do so.
The mass availability of transition-related medical help today compared to the past. It was harder back then to find doctors and surgeons who were willing to prescribe HRT or do surgical work for trans people.
Last but not least, the slow breakdown of gender inequality, as well as the disintegration of gender roles and expectations, has given people the freedom to break gender norms. In the past, gender roles were very ingrained in society, and anyone who broke such roles were often shunned.