It would be worth checking thyroid levels. Our 12.5yo basenji was having similar, but not as severe sounding, issues going up stairs or jumping up on the chair. A diagnosis of hypothyroidism and medicine for a few months solved all those issues. We too thought it was just old age but then noticed some bald spots on his tail and ear tips and got his thyroid levels checked. The vet said he was likely deficient for long enough that it started affecting his neuromuscular system.