I know a lot of the main operators these days require registration to listen via their own apps and websites, so (as long as the listener registered truthfully), they generally have an age, a gender, various demographic information about the listener. They can also tie that in with third-party data from advertising and data collection partners, all those online cookies are there for a reason, they can tie the listener's account with a profile from a data broker to say "this is a 28-year-old woman, married with one girl, recently looked online at Disney vacations and new cars", which is more valuable than just a vague age group from Nielsen.
A lot of stations will then replace the over-the-air ads with personalized ads, aimed at the user's individual location, gender, profile.