"It is true that my grandfather is more than seventy years old."
Translation:È vero che mio nonno ha più di settant'anni.
I don't understand how that impacts my request. The simple fact is that it is listed as correct... in more than one Italian grammar reference. Here's another: "Per alcuni nomi di parentela, per esempio, 'nonno' e 'nonna', l'uso e' oscillante, nel senso che si puo' avere o non avere l'articolo: "la mia nonna o mia nonna" - Grammatica Italiana Con Nozioni Di Linguistica, Dardano/Trifone, 1999. That's not too old, either.
The software should accept grammatically correct answers. If Duolingo wants to suggest a preferred solution, they can do so. But they should attempt to accept all grammatically correct solutions.
Interesting opinions in this discussion and I can see both points of view. Personally, I am more than happy that Duolingo has taught me to stop putting the article in such phrases. If it is no longer in common use why bother learning it? Languages evolve and time-expired phraseologies need to rest in peace.
I've also never heard of the article being used with nonna/nonno...I've heard it's optional with mamma and papa'...and it's only used with loro and further removed relatives (like fratellastro, etc.)... And anyway, I think it's best to learn good habits and go along with what's currently considered correct.