Why is HPV vaccines targeted for preteens?
A lot of sexually active adults carry one or another strain of HPV but most do not develop cancer. You can carry HPV — and spread it to others — without showing any symptoms yourself:


  HPV infections are so common that nearly all men and women will get at least one type of HPV at some point in their lives. Most people never know that they have been infected and may give HPV to a sex partner without knowing it. Nearly 80 million Americans are currently infected with some type of HPV. About 14 million people in the United States become newly infected each year.


There is less reason to give a vaccine to someone older, generally, who is almost certain to be infected with some form of HPV. A vaccination is not a broad cure, though it can provide some protection against infection from strains of HPV that someone is not already infected with:


  But the vaccine also offers some protection for HPV-positive women, reducing cervical lesions by 17% and genital warts by 35%. HPV-positive women usually aren't infected with all four of the targeted strains. In clinical trials, those infected with one or more of these strains before vaccination were protected against the remaining ones.


Pre-teens are unlikely to be infected with any forms of HPV, having no or much less sexual contact, and so they are the best candidates for being administered the vaccine. 

This cohort will get the most protection from those strains that the vaccine protects against, strains which are most likely to cause relevant cancers.