So I'm relatively new at this, so bear with the dumb question. I want to brew with ECY 14 but its not available at the moment. I have some from a split starter in the fridge, but its close to 6 months old. IFfI were to revitalize this through a starter (or series of them) what's the issue? I know people rebuild yeast from dregs, seems like a similar idea to me.
If you can "wake it up" then it should be fine. I have done starters with 6 month old slurry from the back of the fridge and had no problems. If you do this, make the first starter small and low gravity since you probably don't have a lot of life in your slurry.
“What's the problem with old yeast?” The short answer is that old yeast has a lot of non-viable (i.e., dead) yeast cells. Worst case scenario is that all of the yeast cells are non-viable. Try making a step starter like @wspscott posted. If you see activity in the first, small starter that means there are some viable yeast cells which can be further propagated in subsequent starter(s). Cheers!
I do not think this will be your yeast's case but this research is verry interesting on the long term effects of yeast mutation when reusing yeast : http://youngscientistssymposium.org/YSS2016/pdf/Lawrence.pdf
To add on to what was said above, old yeast is essentially under pitching. There are more negative effects with mutation, but here's a link to generally accepted effects of low (and high) pitch rates: https://www.beeradvocate.com/community/threads/best-pitch-rates-to-produce-a-clean-ale.71473/
