Comment by zerocrates

The early recommendation age just falls out of the data that shows the vaccine is substantially more effective if you haven't been infected yet, together with the fact that it's a multi-dose vaccine where the second dose comes months later, and realistically for many that's going to mean a year or more before completing the series.

I think there's truth to the idea that the specific 11-12 range is somewhat arbitrary: as much as anything it's that because there was a preexisting "slot" in the vaccine schedule at 11-12. The American Academy of Pediatrics differs from the CDC's panel on this... but on the earlier side: they would start the recommendation at age 9. I think to a significant degree the thinking there is that if you go earlier the messaging and reaction is more "your child will probably eventually have sex and this is an effective time to give the vaccine" and less "your child will be having sex like, tomorrow."