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.
Yes, it's weird to say that. To be honest, I have never heard someone say that, it's more common to just say "la edad de mis abuelos", because it refers the age of both grandparents without being specific.
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.
Reading this reminded me that we have a paternal grandfather and a maternal grandfather. So I hope to learn too whether 'abuelos' can mean one's 'grandfathers'. (It would be a legitimate sentence such as in a discussion about grandfathers in general.)
In your English sentence there needs to be an apostrophe: "grandfathers' ages".
The exact same ambiguity exists with padres, hermanos, niños, hijos, tíos, sobrinos, esposos, reyes, etc. and I have yet to find any universally accepted way of distinguishing them collectively.
So I usually just refer to them separately and specifically, eg la edad de mi abuelo paterno y mi abuelo materno.
Sometimes you can get away with male/female (varón/hembra) eg mis hijos varones (my male children) but it often provokes weird looks from the unsophisticated and doesn't work across family tree branches, eg mis abuelos varones comes across as my male grandparents---as if all four of them were male lol.