I believe that black text on a white background is universally the easiest to read. Using darker backgrounds can make text harder to read, even if the text is very light.
From my experience, older audiences prefer less colors on a page, but that doesn't mean that a designer has to stick with black and white. Grey, navy, dark red, or other muted tones that are darker would work just as well.
If your site is going to be primarily for older audiences, I'd definitely stick with a white background and black text, but create a little contrast by adding headings, captions, titles, etc, in different colors. This will help break up content and add a little visual stimulation to the page without over-doing it.