I think that in Italian these things are often inferred from context, espcially with body parts. For example "spengo gli occhi" is literally "I close the eyes" but more usually "I close my eyes", because you're seldom closing anyone else's. I suspect it may be the same with family members: just calling him "nonno" implies that he's your grandad (or the grandad in your family), because otherwise you're be calling him "Signor Tagliavini" or whatever, not "nonno".
I think the Duolingo course sometimes insists on the literal translation. Especially then, when the alternative translation would be said differently also in Italian.
"e' vecchio" - he is old "e' un uomo vecchio" - he is an old man
If you translate "e' veccio" as <he is an old man>, then how would you translate "e' un uomo veccio"? Also as <he is an old man>? This means, the translation looses expressiveness. If you translate them differently however, the translations can be as detailed as the original Italian text.
I am not sure if "e' un uomo vecchio" is used by Italians. If it is seldom used, maybe <he is an old man> is a better translation for "e' vecchio".
Excuse grandad he's old. Hmmmmm. DL you mark this as wrong and tell me I should have written granddad with 2 'd' s. I'm trying to learn Italian not English! It is correct, that granddad can be written with double-d in the middle..... If you are Victorians. I don't know of anyone in England who doesn't spell it with one d and I have asked around! Chambers the English bible of English spells it with one d with an extra d in brackets signifying a usage that is more or less dead.
In Italian you do not need to include the subject in front of the verb when it is clear like that. You might want to put voi in front of the verb only if you were really trying to stress that it was it you all who are excusing, and not somebody else, or to disambiguate in some other way.