I was 30 when I was diagnosed and have only ever smoked one cigarette in my life - was super drunk at a bar one night when I was in college and thought it would be a good idea.
I've actally read that there are peaks in the late teens through late 20s-early 30s then again later in life (around 60s, I want to say).
And after a quick google search, here's a link:
http://www.nlm.nih.gov/medlineplus/ency/article/000250.htm
Since the next peak starts again at 50, you weren't too far off from that at 48.
But, I wouldn't be surprised if your quitting smoking triggered your first flare. Who will ever know if you wouldn't have ever gotten it if you had never smoked or had never quit smoking.