It's not just a matter of when you start HRT, of course. This is something that can vary greatly from person to person. I'm pretty sure there's nobody with a final answer to your question. I can say, though, that I've seen photos of many TG folks who started hormone therapy at different ages and I've never seen anyone who started before 20 who didn't look entirely feminine as an adult. It certainly often appears that they have wide feminine hips, too.
Just remember that most genetic females would rather have hips on the narrow side over wide, anyway.
~ Lyric ~