Sometime back when I was playing with a microscope, I took a year old package of Wyeast that had somewhat swollen out of the fridge and did a cell count and viability test on it. I got over 99% viability, so I called Wyeast to see if that was even possible. They said absolutely.
Yeast are a lot heartier than we think. Usually, the older the yeast, the more time to wake from dormancy.
The problem for most of us is maintaining a clean enough environment that nothing else which grows faster takes hold first.
Since then, I've used lots of old yeast as a LHB used to give it to me free. My advice...make a starter and taste the decanted starter wort before you pitch. Do it even if the yeast is fresh. You'll get used to the taste of green unhoped beer and will be able to easily identify when something is not right with the starter.