I don't keep my yeast past about 4 or5 months. I always do a starter. If it's more than a month old, I do a starter in 2 steps. If it's more than 2 months old, I usually do 3 steps. The first one or two steps are usually sluggish. I always want to pitch at the peak of health. I have used 6 month and older yeast, but in my opinion the work and risk is not worth it.