That's precisely the point. If you give it to people at the age at which they're contracting HPV, the vaccine won't be effective.
In fact, this vaccine even when it does work will only at best protect women from four of the twelve strains of HPV floating around.
Actually, there are more than a hundred strains of HPV. The vaccine protects against two strains that cause the vast majority of genital warts, and against two strains that are responsible for more than 70% of cervical cancers. It's not a perfect solution, but it will make a difference.