For children between the ages of 9–14 years, vaccination effectiveness is reported to range between 74% and 93%, decreasing to 12% to 90% for 15–18 year old adolescents.
According to Wikipedia. Effectiveness definitely goes down fast. Also crazy range. 12%-90% sounds like "could be anything". But yeah. Mostly a reminder to pay attention and have your kids get the vaccination on time. Doesn't say a lot about adults. I mean it's safe. And if insurance covers it, why not?
Edit: The linked metastudy has many more numbers. Seems the vaccine is super effective for young people. But most times there isn't any statistically significant effect left past 18 or 20yo. Sometimes they even check for sexual activity... It didn't matter either. Some other few studies still show some effects. But that's probably why there isn't any recommendation to take it. Vaccines only get a recommendation if they have some statistically significant effectiveness. And the numbers for adult groups are just all over the place, whether they take the vaccine or not.