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.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8766261/#:~:text=Gender%20dysphoria%20history%3A%20Of%20the,among%20their%20earliest%20childhood%20memories.