Transgender is (like all letters in the LGBTQ+ acronym) an adjective, not a noun. The word describes a person who has the sustained experience of gender dysphoria.
Gender dysphoria: noun
A condition in which somebody feels that they were born with the wrong sex (Oxford)
Gender dysphoria can be experienced in a number of ways, and the word transgender is often used as an umbrella term for all gender nonconforming, queer identities. Which means that transgender men and women, as well as nonbinary, agender, or genderfluid individuals may all identify as trans. This distinction is important when looking at data about trans youth.
Only about 0.6 percent of the total population identifies as transgender (this number is self-reported, however, and demographers think the actual number is likely up to two times higher). Among youth ages thirteen to eighteen, the number who self-identify as gender nonconforming or trans is 10 percent.
From “Welcoming and Affirming: A Guide to Supporting and Working with LGBTQ+ Christian Youth” by Leigh Finke – Broadleaf Books