[teawithmisswilliams]

If it is most effective at ages 8-12 then why do they begin administering at ages 11-12?

It's not "most effective" then. According to Merck's research, it can be administered any time between ages 9 and 26 and have the same efficacy.

[lqclamar]

It's not "most effective" then. According to Merck's research, it can be administered any time between ages 9 and 26 and have the same efficacy.

Bingo. So why not wait until 18 and let her make her own choice as to whether she thinks she needs it?