In Designing for Senior Citizens | Organizing Your Work Schedule (UXmatters, 2010), several "experts" including Dana Chisnell, Steve Baty, and Pabini Gabriel-Petit discuss the issue of designing for senior citizens. The article references original sources at the end.
Specifically, they mention legibility through color and typography usage. They mention contrast and sans-serif as key choices. They also discuss making it easy for users to adjust colors and font sizes by adding controls to the site that make this possible. They also talk about heuristics they developed: "New Heuristics for Understanding Older Adults as Web Users".
Web Usability and Aging (usability.gov, 2008) specifically talks about the effect of navigation and usability:
More interesting then font size, results from their research demonstrated that the most important factor for increasing usability for the elderly is the use of consistent and persistent navigation.
Main guideline categories of the checklist included: designing readable text, increasing memory and comprehension, and increasing ease of navigation.
The US National Institute on Aging has tips for Making your Website Senior Friendly. This article provides a general outline of what to pay attention to when designing for seniors but has a long list of references towards the bottom that you may be able to use (also referencing Chisnell et al, which suggests to me that they did some pretty meaningful work in this area).
Dana Chisnell is on Twitter. Perhaps you should ask her if she can provide you with some references and resources? I'll also ask her to answer this question.
Overall, it looks like most tips consider contrast, large sans-serif typography, simplicity and control over font sizes to be of major importance. But then I suppose these things are always good design advice.