Nobody can 'change sex'. One is born with either XX or XY chromosomes, and nothing can alter that.
One can dress as one wishes, adopt the appearance of either sex and have whatever sexuality one prefers.
But even a 'sex-change operation' can't alter one's sex.

If, for example, a person born with the male sex decides that he is actually female, that doesn't give him the right to enter private female spaces such as toilets or changing rooms. Or to be assigned to a female prison.
The same would apply of course to a person born female declaring they are a man and entering private male spaces.

I think 18 is quite early enough for a person to decide they wish to present themselves as the opposite sex. Young people are often 'in a state of flux'about their identity, and at the mercy of social media which promotes all sorts of fads and ideas.
If they are administered with hormones or even operated on, they may regret this later, having been rendered sterile for life.