Bette, it is harder to get into med school if you're over 40, but it's not impossible. Some schools are more receptive to older students than others. My school has a reputation of being friendly to non-traditional students and in my class of 60, we have a 43-year-old and a 50-year-old, as well as at least 10 of us in our 30's.
However, I'll be honest with you- the older you get, the harder it is to get in. Still, if this is what you want to do, you will never know unless you try.
Do everything you can to maximize your chances. Consider that you may need to retake some of your basic science classes and/or take additional upper-level science classes, especially if it's been a long time since your science coursework. Make sure your science grades are top-notch and your MCAT's solid. Your clinical background sounds impressive and that will work in your favor. Try to spend some time working with practicing physicians so you can really get a feel for what it's like and have something to discuss at your interviews. The most important thing is to have a cogent, convincing argument for WHY you want to jump tracks and become a doctor later in life.
If all else fails, there are always the Caribbean med schools. Here's a truly inspirational story of a 57-year-old grandmother who ended up taking this route, since no US school would take her:
http://www.tennessean.com/sii/00/10/08/doclady08.shtml Hope this helps and good luck!