Looks like the number of pixels is the most important factor. Personally, I love my multiple monitor setup, but this does make sense. (I'm forty and don't see hugely well for the really small pixels, though.)
"there's an optimal number of pixels you need to complete the tasks you need to complete. Worry about that number, not the number of monitors you have. That optimal number, for the vast majority of people is about 2500x1400. In 2003—before widescreen became commonplace—it was the case that 2 17-20"(2560 pixels wide) LCDs was the only affordable way to acquire an optimal number of pixels. Today, you can pick up a 27 inch display with 2560x1440 pixels along with a computer attached to it for under $1500. This number of pixels allow you to accomplish most tasks—whether it's writing code and debugging, writing a blog post and reading primary sources, or editing one spreadsheet with data from another."
If you're using Windows 7, this is a great list. If you're a system administration you MUST READ this list as there are many reasons in here to upgrade to Windows 7 including the problem recorder feature.