I didn't see the former thread, but will say this.
Teenagers tend to think they understand how the world works, but, in truth, most teens have only ever lived within a very small bubble that consists of school, home and safe slices of the world, curated by parents, where pretty much everyone they meet is of a similar age and roughly similar values.
Once you enter the adult world, usually when you start working in a job, you realise that the world is nothing like you thought it was. It is vast, practically everyone is older than you, and no one gives you any allowances for youth or inexperience, nor will anyone tell you how to operate in the adult world. You will just be expected to do it.
Adult life is difficult, and most people make mistakes in their twenties and thirties. Indeed, most women I know believe they didn't actually become themselves until they entered their 40s.
With all this in mind, part of the reason I would urge caution is because teenagers really have no idea whom they will become in the future. I know women who wanted to be doctors that ended up dancers, writers that ended up social workers, musicians that ended up writing games. There are so many communities in this world, and you want to retain your ability to access as many as possible.
Another reason is that if you transition, you will start your adult life as a medicated individual with developing physical health problems and the possible need for medical interventions throughout your life. Generally, they advise a hysterectomy after five years on testosterone. Are you willing to put yourself into early menopause so young? Freezing eggs isn't the solution posited either. I looked into it a couple of years ago when I had fertility issues and the success rate is not very heartening.
And there's another point to consider: the world changes. And it doesn't always change in the way you expected as a youth. I'm in my early 40s now, and was a teen and young adult at a time when the prevailing youth cultures were very libertarian and individualistic.
I've watched as British society and culture has become more and more conformist, in ways I would have thought impossible, over the last twenty years. I've experienced the ground shifting under my feet economically and politically. Shit changes; decisions you make in this climate may not make sense in another.
I say this because there is a danger that, in twenty years, the transgender generation will be as much a historical oddity as the grunge generation. The transgender idols of today will, over time, disappear, and when you read a "where are they now?" piece when you are in your early forties, you will realise a lot of them were frauds anyway.
This is why so many mumsnetters urge caution. We've seen it happen before. We've experienced something similar ourselves and seen it happen to others.
Be smart. If you have a strong sense of a transgender state, manage it in a way that avoids surgery and drugs. Don't do anything you can't easily undo.