It is bloody confusing. I don't blame you.
The equality act gives gender reassignment as a protected characteristic. And basically it starts as you say it out loud. That means you can't be discriminated against on the basis of it.
The equality act does allow you to discriminate against men, tho. As long as it is a proportionate means to legitimate aim. Like all women shortlists, for instance.
Although the Labour Party are disregarding this, and acting as though anyone who applies already has a GRC.
If a man changes legal sex and gets a GRC he is protected as a woman, because he changes sex to that of a woman.
You can still discriminate, because the law recognises that you haven't literally changed sex.
A rape refuge for instance can keep men out.
As again, it is definitely recognised as a proportionate means to a legitimate aim.
But it's not compulsory, it's something that people can choose to do or not.
And therein lies the problem.
Services have been intimidated/brainwashed, into thinking that they don't have a choice.
As it has not made a legal challenge, so far, no one wants to stick their neck out.
That's my basic understanding.
But As the equality act and the gender recognition act conflate sex with gender, and seem to contradict themselves, it's up for interpretation.
In terms of being under 18, as far as the equality act goes, whatever is written applies to both adults and children. As far as I know.
Which is why you've got teachers being disciplined for misgendering.
I don't believe it has been legally tested as to whether misgendering equals discrimination. Or when it does, or how many times, etc.
Although there was a man who was taken to court for misgendering a TW, but the judge threw it out.