@ButterflyHatched In the nicest possible way, it is extremely naive to think that transitioning young will allow someone to be truly seen by the rest of the world the way they see themselves.
The "milestones" that teenage girls and young adult women go through are not related to superficial things like wearing nail varnish and makeup. If they were, all girls would do these things.
I cannot think of a single experience that has made me into the woman I am today - not the person I am today, the woman I am today, which isn't directly or indirectly related to the fact that I have a female reproductive system and that society by and large expected me to bear children one day. This would have been the case even if I had never managed or even tried to have children.
I don't know what these "missed milestones" are that you think a boy is going to experience if he is allowed to transition to being perceived as a girl at a young age.
An adult trans person, if they feel there are no changing rooms or showers at the gym which they can use without triggering their dysphoria or causing distress to others, can opt out of these activities. They can get changed for their activity before they leave their house, and get showered and dressed when they get home. It's not ideal but the point is that it is usually possible for an adult to avoid washing and undressing in front of other adults.
Children are in school, where PE is compulsory. Schools can't afford to kit their buildings out with lots of individual showering and changing facilities for children who identify as the opposite sex. And it is very clearly a massive safeguarding risk for children to be allowed to shower and change in the company of children of the opposite sex. The child in the Guardian article says they could have avoided the bullying if they had just been treated as a boy from the beginning, but HOW? A 13 year old female student can cut their hair short and wear trousers and call themselves John, but when they take their clothes off they are still going to have a vagina and budding breasts, rather than a penis. Even taking puberty blockers will not alter that reality.
Telling children that if they transition early they can avoid a lot of "transphobia" is dishonest and harmful. Even if they manage to successfully "pass" as an adult, I think most non trans adults would not want to date a trans person. I am not saying this to be unkind. Our sexualities are innate. If you're attracted to men, you probably don't want to date a man with a vagina or a woman with a penis. People can't help this. It's how we're wired. And of course the evolutionary reason for this, and the reason that gay people are in the minority, is to ensure the survival of the species. Most people want to have kids one day. Even a lot of gay people want to have kids, and thanks to modern medicine they now can, but they will still need a gamete from a member of the opposite sex.
I feel for trans people and the difficulties they clearly experience in life. But transitioning young isn't the way to ensure that your adult life will be easier, in the majority of cases. All it means is that children are encouraged to make often irreversible decisions about their bodies that they later come to regret.
If you really are determined to "live as" the opposite sex as an adult then I can see how "passing" would help enormously. But pumping thousands of confused young children with puberty blockers in order to help the small number of them who would never desist even if left well alone to "pass" as adults is barbaric. The other children, the ones who would almost certainly grow out of it if adults stopped encouraging them to believe they are the opposite sex, are not acceptable collateral damage to help people like you better "pass" as adults. And it's not even clear that it does help. Jackie Green was on puberty blockers from the age of 12 and had surgery at 16 and I can still tell, even in photos.
Honestly, I'm appalled at the idea that safeguarding for all children should be thrown out of the window because some adult trans people believe that if they had been allowed to socially and medically transition at a younger age their life would be much easier now.
And someone needed to sit down with that child in the Guardian article and gently explain that even if you force everyone to use their chosen name and pronouns, even if no reference is ever made to the fact that they were born a girl, even if there was no bullying whatsoever, it still would not be possible to treat them as a boy in any situations where we actually distinguish between boys and girls. That child would most likely have had a much easier and happier adult life if her parents and teachers had simply accepted her as a girl who liked to wear trousers and play sports.