The tanner stage is more focused on what's going on on the outside of the breast. Not everyone fits the tanner scale well due to the large variation in breast shape and size. What's really important is what's going on inside the breast. Generally you can safely guarantee a tanner stage 3 for most transwomen, but as others have said the younger you are the more likely you are to hit all 5 stages. I started at 19 myself, and I'm 2 months away from a year and I'm in between a tanner 3 and tanner 4. I tend to think if you start before 25, your chances of having fully mature breasts are higher. Starting around 18 - 19, is probably the best chance of tanner 5 as the younger you are the better the chance. But it is a multi-year process. You may speed through the first 2 tanner stages, and then the next tanner stage takes months to years to get through. I hit tanner stage 3 roughly around my 5 - 6 month mark I want to say, but then it is going to take me until my year point before I can say I'm tanner 4, and then from tanner 4 - 5 may take a few years. It's more important to have the right balance of hormones (I even suggest bio-progesterone just for the added benefits it can give), but I wouldn't really worry about the tanner scale because, you're growing boobs girl, just be happy to have them and enjoy the ride.
