I make my own ACV. Some are five years old when I start using them. The mothers look fine. I have never known a bottle to go off.
A simple "test" you may consider. Buy/make a little apple cider. Put it into a bottle with some of your "old" ACV.
After a couple of weeks at room temperature you should see a new mother growing and have another supply of ACV. And it may taste even better than the supply you currently have.