My wife had a total knee replacement 2 years ago aged 41. She damaged it 10 years prior playing netball and had lived in constant pain.
Since having the operation, she has, with a huge amount of work, gained 110 degrees movement and says that it feels as good as new. She can cycle and walk good distances now. She wishes that she'd had it replaced in her thirties as she now feels that she lost 10 years.
We know that they don't last forever, but following much research and consultation with many surgeons, the most likely failing of the prosthetic joint is loosening and not the actual wear of the surfaces. Impact must be avoided for these reasons, but cycling is OK. In fact it has helped her gain the 110 degrees of movement. She has regular reviews and at the moment the surgeon is saying that 15 years plus is a likely life expectancy for the present joint if she continues to look after it. These things are developing all the time so we hope that in another 13 years, there will be something even better.
Experience seems to suggest that choosing the right hospital / surgeon is important. After a lot of money had been spent on consultants, they nearly agreed that The Royal National Orthopaedic Hospital at Stanmore is the place to go. It's a bit of a shabby Army Camp type of place, but the results are what counts and they have been first class. Interestingly, when my wife was there, most of the patients on her ward were having remedial surgery on for botched operations carried out elsewhere! One lady had spent twenty thousand pounds on a private knee replacement and it had been fitted upside down!