I think they do it so early as they find the vaccine works best before ANY sexual activity has taken place (and in the vast majority of children that will be the case at 12) I'd not heard its only good for 6 years though? I was under the impression it is a one off and if given early enough can prevent HPV as well as warts and a few other female genital cancers.
In fact, having just looked on the Gardasil leaflet that comes with the vaccine, they are saying boosters not recommended, just the 3 parts to the vaccine. Minefield