How your body reacts to HRT is a very individual thing and age, while having some bearing on the matter is not the be all and end all. You are young for someone undergoing gender reassignment HRT. You won't get results immediately but you should expect something reasonable by 9-12 months. Maybe but maybe not. Unfortunately you won't know until you know. I'm 48 and have had a much better response to HRT than I did when I tried it when I was 25, so who can say.
HRT is not a panacea. Passing is not just about hormones, it will come down to how you present, dress, walk, talk, act, plus body hair/beard removal and your confidence. You are an attractive young woman, I don't see why, with the right presentation and confidence you shouldn't pass regardless of HRT.