So I am in the similar situation because I have 5-6 packs of yeast that are around 6 months old. According to mrmalty, viability is 10-20%. A single starter would probably result in underpitching according to calculators, which is why I do two step starter. What I do is calculate as if I start with around 10-20% yeast, and do a two step starter. For example, I needed around 200B cells, and according to mrmalty had 15% (15B) viability. So I put that number in brewersfriend to calculate a two step starter.
I make the first one with around 1.5l, and once it's done, cold crash it, decant the starter beer, and wash the yeast of all the dead cells - put a few cups of preboiled cold water, mix it, let the dead cells and trub settle for 15 mins (depends on how flocculent the yeast is), and then decant the liquid that should contain healthy yeast to a sanitized container.
Then I do another, probably bigger starter (3L) with the decanted liquid, depending on the fermentation parameters for the main batch.
Somewhat complicated, but I believe this is the right thing to do to get the appropriate cell count.