I didn’t even know you could volunteer at the age of 50.
Depends on the MOS and branch of service. For critical high-need duties, waivers are much more likely to be granted. Also, the Army and especially the Army National Guard are much more willing to grant waivers that the other services would be willing to do for the same duties.
There are also special rules for people with prior military service who want to come back in. Here's an example of the Army Reserve taking a 51-year-old with prior service in the Navy: http://www.freerepublic.com/focus/f-news/2848310/posts
I do wonder why a social worker would need to be equipped for night vision, though.