"Yo no sé la edad de mis abuelos."
Translation:I don't know the age of my grandparents.
Why not "Yo no sé las edades de mis abuelos."? Abuelos is plural, it seems most likely the grandparents plural aren't exactly the same single age.
I agree. But these kinds of mismatches are common in English, and I must admit "the ages of my grandparents" does sound a bit clunky in informal speech. It would be interesting to find out from native speakers if las edades de mis abuelos sounds similarly awkward in Spanish.
I think what Duo wants us to learn is that Spanish uses only "la edad," even if talking about more than one person, while English uses "the ages" if more than one. Just a quirk to learn.