I am 55 and started Synvisc shots in 2002, and switched about 4 years ago to Euflexxa. (Started having an inflammatory reaction from Synvisc, that does not occur with Euflexxa).
I have ostearthritis but still have cartilage left. So the injections work well for me, and last from 7-12 months, depending on how much activity I have. When I am working out regularly, they wear off in about 7 months. I do not mind getting them at all, they really make a difference, and I don't want surgery if I don't need it.
My understanding is that if there is no cartilage left, these shots don't do much, so the important thing to understand is what is left there. If there is only bone on bone, the shots may not help at all.
I cannot give any advice on the safety/recovery of this operation for someone over 90, but I have 2 lady friends in their 50s who had knee replacements in the past 3 years. The recovery is slow and painful, lots of PT is needed and you need someone to help you for quite a bit.
Hope this is helpful to you.