A little bit of both. While it's true surgeons are hesitant to replace a "young" knee, joint replacements in adults ages 45 to 64 has risen by 10 percent in the past 10 years. Rates of hip and knee implants are the highest in adults between ages 65 and 84.
Surgeons are still concerned about the revision rate for adults having joint replacements at a young age. Enough bone is removed and changes made in the joint structure the first time that a second replacement can be risky. Since the average life of an implant is 10 to 15 years, it's a safe bet that a 45-year old will likely need at least one more joint implant before age 80.
There are surgeons who will replace a joint in a 45-year old under the right circumstances. Most agree it's best to keep active, maintain your strength, and wait.