In most places, being 18 means you're an adult and can make your own decisions. Their support can certainly help if it extends to finances, and acceptance presents fewer concerns from therapists who are going to ask you to plan your transition realistically. In the case of early transitioners (those who block the onset of puberty, like Kim Petras did, parental approval and support is more essential... no one is going to override a parent's views on transition, even though perhaps someday, especially if a reliable test or set of purely physical tests can be made to confirm transsexual status, that too might change.
Approval for HRT is mainly going to be based on whether you can finance the process, and whether the therapist feels it's appropriate to endorse your plan, which mainly ends up being a judgement call about whether you have fully considered this, as well as in some cases resolving any doubts they might have about your dysphoria.
My trans son is 18, soon to be 19.
My ex has concerns that are somewhat delaying his getting top surgery. Some of that has to do with his (my son's) atypical notions of his identity, and her concerns he is being impulsive (by her sense of things, at least). Hopefully she's going to follow through on the plan to have her own sessions with the therapist and come to grips with and distinguish between her legit concerns, and those reservations that have more to do with an inability to fully empathize... also, our discussion of a few days ago revealed to me, at least, that my son had not managed to get through to her how long he has felt this way... she was unaware, for instance that even his earliest sexual encounters with a presumed female partner had centered on him perceiving himself as a gay male. I understand the lack of getting through, as a similar pattern of expressing anxieties and not inviting a full conversation were part of what drove us apart and led to her unilateral decision to divorce.
