Studies show total hip replacements have been done in patients as young as 16 years old. Usually doctors prefer to wait until you're at least 50 years old. The reason for this has more to do with the longevity of the implant than how long you'll live.
Since most implants last between 10 and 15 years and your life expectancy is in the 70s, you could go through two or three implants in the next 35 years. The soft tissues and bone around the joint don't hold up well after that many operations.
The good news is that more and more studies are being done in this area. Doctors are finding out which implants hold up the best. They're also seeing how to improve the implants for longer and better wear.
Your best bet is to make use of drugs and exercise to hold on as long as you can. Keep regular contact with your doctor in case anything changes in this area.