It looked old, but I wasn't sure if that was just because of the wear on it. I had absolutely no idea what the patches meant, so I started to research and found some interesting things (here's the patch closer up).
It turns out that the US Air Force Security Service was founded in 1948 and operated until 1979, when it changed names (so my shirt is definitely no newer than 1979). The interesting part is that it was the cryptographic intelligence branch during the Cold War, made up of the top 0.5 % of Air Force recruits. They intercepted military information from "countries of interest" (Soviet bloc, etc.) through spoken and Morse code sources (they also analyzed US methods to find and correct weak spots in military security, although apparently sometimes these analysis jobs were really a cover for their more covert intelligence missions). To quote Wikipedia:
These jobs, which required top secret codeword clearance, were extremely high pressure and were considered essential to U.S. cold war efforts. Members of the USAFSS were not allowed to discuss their jobs with outsiders — in fact, USAFSS members could not talk amongst themselves about their jobs unless they were in a secure location. Because of their value as targets, e.g. in Cold War Berlin, the capture of a USAFSS member was worth several thousand dollars, their off-base travel was severely restricted. Many adopted "cover jobs" to more easily conceal their real work.Linguistics-related intrigue! Way more interesting than my initial assumption that "Security Service" implied some kind of run-of-the-mill, well, security--as in guarding a base. Also, as a pacifist by nature, the idea of a shirt worn by a spy is more appealing than the idea of a shirt worn by an infantryman.
Here's a bonus factoid: Johnny Cash was in the USAFSS as a Morse code intercept operator in Germany in the early 1950s.