Actually the public stations also have older audiences, not younger. Which is why commercial stations aren't trying to copy what they do.
I don't have access to Nielsen's data, but anecdotally I'd propose that if you pull a young demographic, say Adults 18-34, you'll find numerous markets on the East Coast where the public talker out-rates the conservative talker. NPR talk stations tend to have a lot of very old listeners in their 60s to the grave, but also a healthy "next generation" of listeners in their 20s and 30s who tend to be college-educated professionals.
As opposed to conservative talk, which even conservative young people don't find entertaining.