I just tell people I'm retired. (I was a nurse for 17 years.) I'm 56 so it's not outside the realm of possibility for a person to be retired at my age, especially from a career that demands so much from its practitioners.

I have on occasion said I'm disabled, but of course people want to know why and I'm uncomfortable stating the truth. I've found it's much better to say I'm retired and now have turned my attention to full-time writing, which is almost the truth.