I started transitioning at 33. I was terrified of coming out and it took a lot of mental preparation; in fact from the point of thinking 'I need to do this' to telling my family was probably around 6 months. In my head was the worst case scenario: rejection from friends and family, career suicide, end of my relationship. In truth only the last of these happened and even then she's still my best friend. So generally, my fears were unfounded.
I have found though that my oldest male friends, whilst initially supportive, have slowly distanced themselves. I think that when I was talking to them about what was going to happen they wanted to be accepting and help me through it, but once the reality kicked in I think they've found it too hard to deal with and so have just drifted away. That's only two people though, and I've made many more friends since coming out.
I was lucky in a few ways though; for a start, I'm small and slight and pass pretty well, especially since FFS. I think (and my parents have confirmed this to be true for them at least; they said if it had been my 6ft broad shouldered brother they would've been a lot more fearful) that some people would've been less supportive, or at least more likely to urge caution, if I didn't look like a woman. It's not fair but it's just how people work I suppose. Also, I work in a creative and young industry, where people are generally quite liberal and accepting. And I wasn't married and I have no children, which must make it much harder.
I think though that it's never too late!