Joint replacements are becoming more common as the population ages. It used to be that hips and knees were only replaced in older people. The actual joint mechanism and materials used have a certain life span. If a particular joint has a lifespan of 15 years, an implant in someone who is 70 years old should be ok until that person is 85, for example.
Because of this limited lifespan of the hardware, doctors have been reluctant to use replacements in younger patients, particularly those under 55 years old. They feared that the rate of surgery to fix or replace the replacements would increase dramatically, and that the more active lifestyle of the younger patient would put added stress on the replacements, causing more problems with the hardware.
Research is showing that certain types of replacements do do well in younger patients and that the replacements are a better option for some. Because of this, more doctors are performing more replacements in younger patients.