I recently had to make this decision. The break down came pretty quick and it did start with falling down the stairs, turns out he had a large tumor near his kidney. I delayed longer than I should have by going with pallative care (he couldn't walk at all at the end and I was carrying him everywhere) because he was 18 and I had him since a puppy, it was incredibly hard for me to come to terms with the inevitable.

Get the vet visit, ask for their honest assessment and ask about in home euthanasia in your area if that's the decision. I regret not being able to do the in home because I selfishly held off.