I don't want to assume, so I'll ask if you're willing to share - are you making the implicit assumption that your kids are and/or will be monogamous, and is that assumption a key factor in your decision on their vaccinations?
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."