This is a bit of a "You can't transition because your life will become too difficult due to the way I am going to treat you." classic here.
Transitioning can help ameliorate debilitating gender dysphoria. This can be addressed in many ways - some of them are connected to an individual's own self-perception and others can be due to how they are perceived by others.
Transitioning won't completely change the way others treat people they perceive to be transgender. This itself can sometimes be a source of dysphoria.
Sometimes transitioning can cause people to be treated as if they aren't trans at all. This doesn't fix the problem of societal transphobia - it just sidesteps it for that individual.
Transitioning young increases the likelihood of a person having a fighting chance of living a life exposed to the least possible amount of prejudice, and means they can potentially do so before they become an adult. It helps avoid a sense of 'missed milestones' and the trauma of experiencing gender incongruity for a large portion of your life. It isn't a panacea but it can help some people a great deal.
This privilege - to appear to be an ordinary person below the notice of those who systematically abuse transgender people as a matter of principle - once gained, can be revoked at any time. You only get to have it once in any social group. Some people don't take advantage of it at all and always disclose their history anyway.
Minority stress remains a lifelong factor regardless.