I think it's common to conflate Gender Dysphoria and sexual maturity since hormone therapy is a common treatment. Here is a study from the nlm about the age transgender adults first experienced symptoms of GD. It's only one study so maybe do some additional reading.
The age at which transgender women (TW) and men (TM) first experience gender dysphoria (GD) has not been reported in a U.S. population of adults seeking genital gender-affirming surgery (gGAS). Because gender is an innate part of identity, we hypothesized ...