Approval vs works. Medications & vaccines are tightly regulated by the fda & the petition for approval & use for a specific product follows the paperwork required. The company comes up with a target age group & must collect data how effective it is in that age range for review & approval is granted for an age range. The vaccine likely works well from early childhood to old age, but the extra cost isn't worth it.
Age of sex exposure. Gardasil is a vaccine to prevent cancers and warts caused by 4 strains of hpv virus. It is approved for females ages 9-26 years. Recently, it got approved for males also. Most genital area hpv infections are from sexual contact. Researchers had to pick an age range, so they picked the years in which people are more likely to have sex with several people, prior to marrying and becoming monogamous.