You don't really change your sexuality, you just figure it out as you go along. And same with gender.
My biggest mistake was taking so long to figure it out, so it just depends on the person. Not many people find they made the wrong decision, since I read about a person who asked a hundred cis people to take hrt and most of them were disgusted by the idea, so if somebody wants it it's probably the right decision.
If you wait to change until you're older, you can spend years stuck with dysphoria (which a lot of people don't properly describe, it's emotional pain so strong you can feel physical pain from it and want to give up on life, lie on the floor, and cry until you fade away to nothing).