if you get a hip replacement before you're 18 how many times in your life will it have to be replaced?

How often depends partly on how you treat it (engaging in high-impact activities is one way to wear out a prosthesis prematurely); partly on what materials the bearing is made of (if it's ceramic/ceramic it might wear more slowly than, say, metal/poly); partly on how long your life is; and sheer luck. Add to these things the possibility that new developments in artificial joints may extend the life of prostheses in the future.

I have both hips replaced with metal/poly bearings (I was much older than you are when I got them) which have a failure rate of about 1% per year. That means that in 15 years, 85% of the hips are performing well and 15% have failed for some reason. No one can predict exactly how long it will be before you will need revision.

Depends on how long your doctor says you have until it has to be replaced again. My rod had to be replaced every 10-20 years and I was 15 years old. That would have my rod being replaced on an average of 5 times if I live to be 90 years old.