Baby boomers who think they're not mired in nostalgia the way oldies fans are, because they're seeking out new music. But the new music they're finding is done in styles most appreciated by baby boomers. To most of today's 30- to 45-year-olds, the acts played on AAA radio just sound old, even the ones that feature younger musicians. Nathaniel Rateliff is a perfect example. If you liked Gary US Bonds or early Springsteen, you probably like Rateliff. But listeners whose youth was full of grunge and early hip-hop aren't going to appreciate him.
It depends on the station, there are different flavors of AAA. The non-comm stations like WFUV New York fit that description because they depend on listener contributions, and I think public radio in general gets most of its contributions from older listeners like those people who love Springsteen. But a commercial AAA like KBCO is going to target a younger audience, still firmly in the advertising demo. If you look at their playlist it's not nearly as focused on the folk/roots/classic rock styles. It's modern, melodic, alternative-leaning acts like Lumineers, Death Cab For Cutie, Cage the Elephant, The Black Keys, Mumford and Sons, etc.