Overall I've read that the younger you start, the better off that you will "physically" pass but this is also dependent on your genetics and how masculine/feminine your body/face is. I've seen videos of people in their 40s starting Hrt and still passing so there is a lot of room I would say. From what I've heard it wouldn't make a big difference for me if I started when I was 18 instead of when I was 20 but honestly I still wish I did

Good luck and best wishes to you!
<3 Emma