I was 39 when I got it. It took about 6 months to develop from no symptoms at all to really bad. Before the first symptoms I hadn't had any problems at all (and I have been programming since I was 15).
In retrospect, I should have done much more as soon as I felt the first symptoms. However, I didn't really know what to do, and I was sort of just hoping that it would go away by itself. Then I went to see several doctors, but none of them could help. It was only when I started doing my own research that I found effective ways to deal with it.
Also, it seems pretty clear from the research I did that not everybody will get RSI, even if they work just like I did. But the problem is that we don't know beforehand who will get it. Listening to your own body is a good precaution though. I wasn't very good at that I think.
In retrospect, I should have done much more as soon as I felt the first symptoms. However, I didn't really know what to do, and I was sort of just hoping that it would go away by itself. Then I went to see several doctors, but none of them could help. It was only when I started doing my own research that I found effective ways to deal with it.
Also, it seems pretty clear from the research I did that not everybody will get RSI, even if they work just like I did. But the problem is that we don't know beforehand who will get it. Listening to your own body is a good precaution though. I wasn't very good at that I think.