age can play a difference, but I'm also a strong believer that genetics plays an even bigger role.
I started HRT when I was 26 (I'm now 31) and I seem to be fine. I know younger girls who started younger and had a harder time with looks or breast growth, and some didn't. I know several girls older than me that have bigger breasts than me and look drop dead gorgeous, others, not so much.
I know that sounds hit-and-miss, but that's how it rolls. That doesn't mean you shouldn't go ahead - and honestly, trying to judge if you'll do good or not before even going on HRT doesn't work. I've known girls who, before HRT, thought they'd never look good and they'd never pass and they'd need massive surgery. They then go on HRT and they look stunning and are hit on constantly. A lot of changes can happen.
Beyond changes, self-confidence in who you are makes a huge difference. You can look amazing, but if you have no confidence in who you are -- namely, a woman -- even with perfect looks and voice you're not going to get far. It takes time to build that confidence and to own it. It can take years, but the psychological aspects are very important and give you a foundation to build upon. What you think is what you become.
So, take that time to psychological be yourself and stick to it. Then, use the tools to build upon it, such as HRT, voice training, make-up, surgery -- all of those things. They are tools, but they won't make you a woman -- only you can make you a woman, and you can only do that by being yourself and owning your womanhood. A lot of TS girls will focus on getting acceptance from others, constantly seeking validation that they are a woman. That's fine, and natural and it feels great to have that validation; however, if you don't validate and accept yourself and own your womanhood, all of the validation from others will be fleeting, and you'll double guess yourself and feel like you've gotten no where. It's about confidence in who you are and knowing who you are.
I might not have the best of confidence sometimes. I find that I'll look at my picture and think I look horrible. I'll think my voice sounds like crap. I find that I will tear myself down sometimes and I suffer from self-esteem and depression problems. But, at the end of the day, no matter how awful I might look, I know who I am and I own my womanhood. I am a woman. Period. Others sense that, and I find it makes a huge difference. I don't get weird looks, I don't get people asking me if I'm a man or a woman, or people calling me "sir."
Yes, it took time to get to this point. It's part of the transition. It's not going to happen over night, but I'm a strong believer that how you carry yourself psychologically in the aspects of knowing -- not wanting to be, or hoping to be, but knowing -- the difference is huge.
The other stuff, age, genetics, are things we really can't always control. I would have loved to start HRT when I was 15, but it wasn't available then. I'm sure most TS girls would say the same. We can't go back in time to change that, but we can do what we can now and own our womanhood. I am a woman, this is who I am.
Use the tools of HRT, surgery, clothing, make-up, what ever you choose to help you on your journey to yourself. But, always own yourself. Be true to yourself. Transition is about becoming yourself, your true self. It's a journey, with a lot of ups and downs, but always keep in mind that you are transitioning for you, not the rest of the world. Always, always be true to yourself. In the end, yourself is all you have.