For a discussion of the origin of the phrase "cut the mustard" see

http://www.wsu.edu/~brians/errors/mustard.html

http://www.shu.ac.uk/web-admin/phrases/bulletin_board/2/messages/198.html

To hear an actual recording of "Too Old to Cut the Mustard" sung by Marlene Dietrich & Rosemary Clooney, go to

http://www.thegardenhelper.com/real~oldies/too~old.html

Actually, I was looking for the words to this song, and have failed to find them so far. I can't play the above recording myself because RealAudio doesn't work on this computer, so if someone else would care to listen to it, transcribe it, and post it here, I would much appreciate it.