The vaccination is most effective at this age. As with most vaccines, such as those that we administer at birth, the younger the person is when they receive vaccination, the better the uptake (sero-conversion or immune response) by the body.
The other reason is that after research it has been established that this is the group of young girls where the distribution of a limited number of vaccines can have the highest health benefits in the future.
The World Health Organisation (WHO) recommends that girls from 9 years should receive the HPV vaccine before they become sexually active. Clinical trials have shown that the HPV vaccine is more effective if the girl is younger.