I had my left knee replaced in 2020, due to arthritis, aged 50.
My experience:
It wasn’t as painful as I’d expected, but I’ve had a mastectomy, chemo and radiation so perhaps my view of pain is a bit skewed. It was certainly uncomfortable but the pain meds work pretty well. It’s no walk in the park, but if you take your pain meds as prescribed, the pain is manageable.
Rehab is absolutely key. The physiotherapy is hard work (and can be very painful) but you have to grit you teeth and get through it. If you work hard in rehab your end result will be much, much better.
Be prepared to be quite disabled for a good while. My walking was slow and my movement pretty limited for weeks. My husband said it was like dealing with a very elderly person - everything you do takes so much longer!
But ultimately, two years down the track, I have to say I wish I’d done it sooner. I have zero pain, my movement is amazing, and I am able to do so many things I couldn’t before. People are always surprised when I say I’ve had a knee replacement, as you would never be able to tell - no limp, full range of movement, very active and strong.