Well, there are various facets to this:
1. metallurgy: we know a lot more about the materials used in implants now, cobalt chrome, stainless steel, titanium, ceramic, polyethylenes. We also know a lot more about the interaction of bearings of same or different materials and the impact on the human body
2. implant manufacture: has increased in sophistication beyond all recognition to what it was 40 years ago! It's very sophisticated and improving all the time.
3. surgical skills: surgeons have a lot more information, improvements, research and training into the technique of joint replacement. They go into every little detail these days, bearings, stress shielding, cement security, implant durability, etc., etc., where years ago they just talked about JR in the general! However, not all surgeons avail themselves of this information and training, so when you are making you selection, ask them how often they attend symposia and when they last attended one. Most all of the surgeons who tell people to wait as long as possible are ones who are not keeping themselves up to date and are still working on old opinions.
It's because of all this that the age limit is gradually coming down. When I was first trained, you had to be over 70 to have a THR! 69 was just not considered so bad luck! There was a lot of nervousness about how long these new flangled things would last. But now we know there are people who've had their joints in for 40 years and even longer. Remember, that means they had their JR done in 1969 when the science was barely out of its infancy! I wouldn't be at all surprised to soon be hearing reports of JR's lasting 50+ years and those that are being put in today - well, I won't be around to hear the results but I think it's looking pretty good!