Usability in web design or usability case study are a few that you can use in Google/Bing to research some of what you are looking for. There are quite a few good books on usability that I recommend to anyone asking about this. Don't Make Me Think and Rocket Surgery Made Simple by Steve Krug are two very good books on this subject.
You are wise and also very lucky to find the amount of people that you did to run a usability test on. Any web design is going to depend on a variety of factors and as you pointed out age is one of them. There are a multitude of guidelines that you need to factor in with each and every design. So many factors that I don’t have the time here to go into much detail.
Here are a few links that I touched on while doing a fast search for you that will help. Although not the whole picture it might give you a one up to know what to search for. If you are interested in this subject as all web designers should be I strongly recommend the two books I pointed out.
http://www.usability.gov/guidelines/
http://www.1stwebdesigner.com/design/case-study-call-to-action-in-web-design-usability/