I've made beer from yeast cake slurry that was over 6 months old, but I made a starter from it. The nose knows. If it smells good and appears to be good (no mold) chances are it is good, but I would certainly make a starter from a yeast cake over 6 months old. Not to say you can't do it because I'm sure it can be done , but I would be concerned about the yeasts viability.