I'm in my late twenties and I know both brands though I had almost zero interaction with Zagat in any purposeful sort of way. I never sought out a list of Zagat rated restaurants. I did use Frommer's for trip planning though often it was browsing in the bookstore or library rather than purchasing.
Interesting, though not entirely surprising. I'd surmise that your usage is representative the maximum utility for most people under 40.
Did you seek them out or did you discover them incidentally while in the respective "Travel" sections?
I'm in my early 20s, and I have heard of (and respect) Zagat. I'd never heard of Frommer's until this story broke.