Thank you, Serious Eats, for educating the world on how to avoid stale chocolate candy!
After reading this article, I immediately ran to my pantry to check the date on the two fun-size packages of M & M's that I have. One of them was a little old, manufactured in the 40th week of 2007. The other one was made less than two months ago.
Oh, and if you are wondering, I do notice the difference in taste.