A lot of replies that are mostly true, or somewhat true, or simply missing the real reasons.

There are two factors here:

1) Vaccine-derived immunity is a function of the individual's immune response, which in general, weakens significantly with age. It is not unrealistic for a vaccine to simply fail to elicit any response in someone old enough.

2) It is very, very difficult to recruit folks without HPV that are over 40 for a clinical trial. Most people of that age, who were never immunized, most likely have had it. This significantly convolutes the signal.

3) This is all especially confounded once something becomes "standard of care". Every year there are fewer and fewer people age 40+ with HPV.

For these reasons, the vaccine is currently officially ??? in people over 40. Most doctors will prescribe it anyways if you ask. It may or may not infer immunity. It almost certainly will not harm you.