You might expect a high level of accessibility from a site called Usability.gov. You might be disappointed. Sadly, I wasn't particularly surprised by my quick look into the accessibility of this site.
Age is definitely a factor. While the copyright is up to date, the fact that has custom style sheets for Internet Explorer (IE) 5 and newer, and 4.7 and older makes me think it's not a new design. With that code, I'd hope it was more than five years old. Regardless, it's showing its age.
Some of the low points of the site are: