In the days before arthritis most people don't notice them much and go about their day without worry.
Then the pain starts, just a bit a first, and many people find going down stairs really bothers them on occasion. After a few flare-ups, people suddenly remember where all the stairs are in their daily life and what changes of route can avoid them.
Next, one knee is so bad that it's up and down stairs (only if they can't be avoided), one step at a time, same leg leading every stair.
Soon, on the bad days, stairs can't be done at all.
This progression is probably familiar to many people and a frequent question when someone is considering knee replacement surgery is..... "will I be able to do stairs again?"
This is very much a realistic goal for the vast majority of people, and with the help of the physical therapy team usually achieved in the first week after surgery. Obviously doing stairs one at a time, with a rail and a crutch is a bit different to walking down stairs like you used to, but time and hard work will get you there. There is a large amount of variation between people as to how quickly they progress, and many get despondent when the patient in the opposite hospital bed is doing better. By 3 months later the field has come back together and everyone who worked hard has a good result.
Remember - the surgeon has finished his job before you even wake up after surgery. From then on.......
Its up to you.