What techniques should I be using when testing of a website for visually impaired users that commonly occur due to aging such as macular degeneration, needing reading glasses, possible loss of contrast ranges, etc.?
Note: The reason I wrote this answer highlighting the different checkpoints a person must do to ensure his site is accessible to people with limited vision is because I believe an understanding of the faults or design issues in the site will help define what the problems users might face while accessing the site
For starters, I strongly recommend considering testing your website to see how it handles screen readers as that is one potential user base you cannot avoid and also you cannot always assume that people might use magnifying tools to read content in case of low vision and screen readers are an expected accessibility tool not only for "blind" people but also people with limited vision. So by avoiding them you are technically alienating your own user group.
That said, there are some recommended practices about what should be done for ensuring visually impaired users can access your website.
I also recommend having a look at this article from Mashable
In closing here are some links worth checking out
Eyesight starts to degenerate from about mid 40s on and by 50 many people will be needing reading glasses.
People tend to resist getting glasses so there's a age group around 45 - 55 who will moan about your site if they can't read it because they don't want to wear glasses.
As always, the best way to test your website, is to recruit some users in this age range and test how they get on with it.
I assume you are asking how to do this manually, after having already built your website? However, it is possible to automate some of this testing while you are still developing. I'm not sure how many people are aware that this is possible, but I hope you (or your developers) find it useful.
An example of some of the audit rules that are included in the reports:
One of the main benefits of automated accessibility testing (as with any automated code testing really), is the ability to pick up if a change in your code weeks or months down the line changes the accessibility of your site in ways that you may not have foreseen.
Another benefit is the reduced workload on your QA teams, allowing them to focus on specific interactions or complex pages.
This may not be a silver bullet, but may give you confidence on at least a baseline level of accessibility going forward.
There are several tools that can be used to verify contrast levels to make sure there is enough contrast between the color of your type and the background it's on. Here's a link to 10 such tools. http://www.456bereastreet.com/archive/200709/10_colour_contrast_checking_tools_to_improve_the_accessibility_of_your_design/
Regarding font size, just make sure you're using EM's to proportionally size things so that users can change their browser zoom settings and your layouts will still look OK.
If your user's eyesight is so bad that they need to use a screen reader, I would recommend doing a Google search for consultants that will help walk you through the process of getting your website to function well for screen readers. JAWS ( http://en.wikipedia.org/wiki/JAWS_(screen_reader) ) is currently the most popular. If you don't have the money for a consultant, download JAWS from the maker's website and play around with it along with reading their online tutorials. Getting your website to work properly with screen readers is tricky business, I've worked on a project before that required it and we had to hire a consultant.