Does euflexxa work for old knees?

Yes and no: It depends on the amount of cartilage you have left. If it is bone on bone it is less likely.

Depends: Knees that are "old" do not necessarily have arthritis, but arthritis does get more common with age. If you have symptomatic arthritis of the knees, one of your options is "visco-supplementation" injections with a medication such as euflexxa. This type of injection can help for 6 months or often longer to postpone surgery. The benefit will be influenced by the severity of your arthritis.