I mean, a 40 year old wants to transition? Fine, you've lived your life in your body and you may know what you want. It's a free country. You do you.
This shouldn't be an issue for 21 year old people. They don't know themselves fully yet. It's the same issue I have with all of these early transitions.
Do what you want with your body and your life. But be absolutely certain before you fuck it up beyond repair. And it's too easy to make serious, major mistakes that can have repercussions for the rest of their lives at that age.