In the US, and probably right across the West, increasing numbers of teenagers are now identifying as transgender, according to recent studies:
  http://abcnews.go.com/Lifestyle/wireStory/boy-girl-teens-identify-transgender-52840089 

“The study looked at students in ninth and 11th grade and estimated that nearly 3 percent are transgender or gender nonconforming, meaning they don’t always self-identify as the sex they were assigned at birth. That includes kids who refer to themselves using neutral pronouns like “them” instead of “he” or “she.” “Diverse gender identities are more prevalent than people would expect,” said lead author Nic Rider, a University of Minnesota postdoctoral fellow who studies transgender health. The study is an analysis of a 2016 statewide survey of almost 81,000 Minnesota teens. Nearly 2,200 identified as transgender or gender nonconforming.

The study found that these kids reported worse mental and physical health than other kids, echoing results seen in previous research. Bullying and discrimination are among possible reasons for the differences, Rider said, although the survey didn’t ask. Rider said it’s a study based on a statewide population of teens in ninth and 11th grades and that the results can be used to estimate numbers of trans and gender nonconforming teens in those grades across the United States.”

     Although it is being said that there were always such large numbers of transgendered people, right throughout history, but only in these enlightened times are they free to be, well, free, there is an alternative view. It could well be the product of media attention which has celebrated transgenderism, which is not to say that many people do have legitimate gender identity issues and do indeed need to be treated with compassion and understanding rather than hate. Nevertheless, the statistics of the rise begs for an explanation, for it is unlikely that such numbers of people were always present in society, but repressed.

     There is a possibility that hormone-altering chemicals, which rage through teenage bodies could be having an impact, perhaps in combination with social causes:
  http://www.dailymail.co.uk/news/article-5351661/Chemicals-plastic-90-teenage-bodies.html

     It has been found that up to 90 percent of teenagers have high concentrations of the chemical Bisphenol A (PBA) in their bodies, which mimics the female sex hormone oestrogen, and has been linked to low sperm counts and infertility in men. The evidence for a hormonal cause for transgenderism though, is controversial:
  https://www.livescience.com/51652-transgender-youth-dont-have-hormonal-imbalance.html

     and is unlikely to be due to any simple hormonal imbalance. My own view, is that the sociologists are right about this one, and that it is indeed a social construction:
  https://medium.com/@jackisnotabird/if-genders-a-social-construct-is-being-trans-just-a-construct-too-f9740bb9f6f