Worried about HRT/Passing

So, I've seen a good amount of trans people saying that taking HRT past the age of 18 will make it extremely hard to pass later on in life (for reference, mostly trans fems. I'm not sure how much harder or easier it is for any type of trans people to transition). I am a 16 year old trans fem with extremely unsupportive parents and no income of myself, so I'm wondering, am I just sort of screwed? I'm probably gonna be able to start actually doing stuff when I'm 19-20, and I've just been really worried that it'll be impossible for me and my transfem girlfriend as well to pass later on in life