The FDA itself restricted access to the vaccine on the basis of age. Given that virions aren't even involved in the production process, its safety should have been deemed good enough for the entire population early on.
Same reason you can't get Shingrix under a certain age.
https://www.boots.com/online/pharmacy-services/shingles-vacc...
> Price per dose:£230
> Full course (2 doses):£460
Insurance companies used to only pay for the vaccine at 60. They've reduced it to 50 now because people (like me) were getting it in their 50's. I got it in my left eye and because my immune system is kinda shit, I still have it, though it doesn't give me too much grief now. But it did trash my cornea in that eye, so it's messed my vision up pretty good. And since there's still an active infection (after 8 years), I can't get a cornea transplant.
https://www.health.harvard.edu/staying-healthy/two-dose-shin...
(1) a booster won't be invented in next say 20 years and
(2) you will live next 20 years (likely if you are healthy and have a healthy lifestyle)
The original trials were for a specific population - no prior HPV infection, young women. Hence the approval was for that population.
Additional trials have been run expanding the population, but the decision was based on data not “yeah, I’ll bet this is safe/works for this other group”