Well I don't think age has much to do with anything. It's diet and luck or genetics. If you're younger, it's obviously better. But, being young in and of itself is not better. For me, I have been on a transitioning does of HRT for six months, and a lower dose for six months before that. But even that changed me. I've only recently begun to see myself as female in the mirror and think of myself as female on the inside too. Rather than a fraud, which is what I used to think, and still do to some degree. I hope that goes away but, come hell or high water, I'll get as close to being 100 percent female as possible, even though being totally cis is impossible.
But like Jen, I really can't say what changed, all I know is that something did and everyone I know has said "So, you're a woman now!" Or something to that degree. I've been blessed with a very femme body, but yesterday I hid that and tried to get on a train with a half-off disability pass that says MALE. The woman told me to take it out and then went, "No way sweetie. That's a man's pass. You can't use that." So I said "But, uh, I'm, um, uh, a transsexual." And she said "No way. Really? Get outta here, girl. OMG that's so awesome you look so pretty. Lucky you! I'm so happy for you." True story. You couldn't wipe the smile off my face. Because (A.) I passed so well she barely believed I was trans. And (B.) she thought it was awesome I'm trans. So, even if you never, pass, there are people in this world who are on the side of trans peeps. Hell, she is prolly more on our side than I am lol