There are five year olds that know for certain they are trans* and the parents sign off on the legal name change then. There are tweens, teens, and adults above the age of majority going through the court system and top/bottom surgery (or deciding no surgery) as well. At what age do we trust cisgender heterosexual boys to identify their gender identity, gender expression, and sexual orientation? Texas courts think it is the same age that a trans* person can figure it out and I agree with them. Now is the first time in history with the visibility and levels of acceptance that trans* kids are able to come out and live out lives when they are ready. The earlier they are ready the more they can benefit from community resources and support, that they can't access from deep within the closet. There are large numbers of Eagle Scouts that hid in the closet until late in life but they too find a great deal of support and resources that did not exist twenty years ago. They might have known they were trans* since they were 13 (or 5) but they never felt able to live their out life until a certain point; just because they didn't get bottom surgery until they were 45 years old doesn't mean that a 13 year old today can't comprehend the various factors and reach an appropriate decision for their life to begin transitioning.

Considering the number of Texas school districts that have fully inclusive nondiscrimination policies against trans* students, faculty, and staff (such as HISD) it's usually the fringe ones in the country you expect to attempt to police genitalia - and they end up losing under federal administrative law since "gender" is a lot broader today than it was in the 1950s. There was one school district in "the country" just outside of Houston that got BTFO on that single point earlier this year, but I am explicitly not going to identify which school district it was on reddit or on private message.