About a month ago I used some WLP004 that was 1st gen washed sitting in my fridge for about 6 mos or so, it was a 10 gallon batch of a stout.
Only thing is that my attenuation sucked, 60.5% AT. Having said that, the best attenuation I have ever gotten with this yeast is 64%, and Whit Labs states the attenuation range is 69-74%.
So I can't say whether or not the age of the yeast has anything to do with it, I did mash at a higher temp, and about 30% of my grain bill were unfermentables...just sayin'.
On a side note, I have more in my fridge that I plan to use, so I'm going to experiment with it.