"It ain't easy to find virgins in that age group,"
The ages tested were not the ages targeted for the vaccine. They were 16-26. I didn't see it mentioned whether they were virgins.
I believe the sexual activity had to be limited to 0 or 1 or 2 partners.
Also, part of proving that preventing infection by the vaccine is effective is that they were exposed during the time of the study. (sex)
No one would want a study in 11 year olds that expected them to have sex, and thus exposure, with in the next year or two.
Yes, those ages included in the study are also within the label requirements for vaccination, but to try to be ahead of a child's first sexual experience, vaccination is prefered before likely sexual activity.