I don't know if there are statistics for that or not. All I know is I have been practicing for one year and I'm 46. My goal is to be in a place where I can continue to work on a part-time basis until I die. Seriously. I think retirement is generally unwise unless a person is going on to do other good works with their talents and blessings. As long as my mind is sharp and I continue to love people, I want to work.
Interesting question. I have been in practice as a nurse practitioner since May 2004, when I was 34. I'm in my 40's, my ICU NP colleagues are between late 20's, 30's, to same age as myself (I'm one of 3 who are in our 40's). I'd like to do something different by the time I'm in my mid-50's possibly teaching or doing research and have been torn between a DNP or a PhD for years now. I intend to work until my late 60's outside of active clinical practice.