It's someone who is underage that doesn't identify with the gender they were assigned with. There's no specific age that it's determined, but there can be signs of transgenderism as early as age 3. I think it's best determined by a medical professional. Just because they are trans does not mean they immediately start hormone therapy, puberty blockers or surgery. Most socially transition to start by changing name, pronouns, clothes, etc.
I'm not sure I understand your last question.