I had a knee replaced 8 years ago when I was 49. I was told it would last between 10 and 15 years. They prefer not to repeat the operation, which is why they prefer to wait until you are older, but if the pain is too much, as in my case, they will consider it.
Since my operation, I have been pain free, not as much flexion in the knee, but that's to be expected. I used to play a lot of squash, that's a no-no now.
I swim a lot, (not breast stroke) for exercise.
I'm glad I had the op, I'd advise you to go for it! Good luck.