The way I understand it is, Once you have been exposed to HPV, the shot does nothing. It doesn't rid you of the exposure to the virus, once you have it, you have it. Since HPV is so widespread among the population, chances are if you've had a few sexual partners you've already been exposed. The shot prevents you from aquiring the virus if you have not yet been exposed, therefore the best people to give the shot to, are those who have never had any sexual experiences. I believe that is the reason for the age range of 9-26. I could be wrong.