The whole "people shouldn't transition before age N because their brain isn't fully formed" thing is so catastrophically dumb.

Like, do you really think I wanted to be a fully formed man? Seriously. What's the upside?

For most trans people, the more of the wrong gender we get, the more pain it brings. Waiting until those changes are permanent only makes them worse. It's like if you have a gluten allergy and someone says they won't believe you until you've tried every loaf of bread in the bread aisle. More pain doesn't fix it. We don't get over gender dysphoria by exposure therapy. We just get more trauma that we have to live with the rest of our lives.

Instead, being able to transition BEFORE the brain and body are fully formed is the ideal. The earlier the better. The earlier you're able to get access to medical transition, the closer your fully formed body and mind will be to what they should be. That's a good thing! Waiting only causes harm.

The only reason why people think like this is because they refuse to believe trans people and trans experiences. They think that their default cis assumptions about people should override those people's own experiences. Sure, they'll grant some allowance in theory to adults who want to muck about with gender. But they don't actually believe we are who we say we are. Not if they think we have to have known since we were 4 but shouldn't transition until 25. The only way that makes sense is if they're either trying to be unbelievably cruel or if they simply don't believe we're real.

If you want to know if someone's an ally, look at how they treat trans kids. Because being an ally starts with believing us, that our experiences are real. And if they won't believe the kids, they're not going to believe you when you're 35 or 45 or 70.