"Old" would be "vecchio" but that sounds rude to Italian ears, so people usually go for "grande" and "piccolo" when comparing ages; you can use "grosso" to explicitly refer to size. Bigger isn't mistaken though, and it's not like it doesn't work in English as well (my big brother, my little sister).
You could also use "maggiore" and "minore" to refer to someone--especially a family member--that is older or younger?
Yes, siblings are often ordered like that: mio fratello maggiore, mia sorella minore. It doesn't really work outside that.
ABSOLUTELY. The grammatical implication is "He is older than SHE IS OLD." "Than her" is grammatically incorrect.
Yes, the use of nominative after "to be" is a running battle. Some don't know of it, others say it's old use and a few maintain that good grammar is always correct. It's an uphill battle. Duo usually accepts both.
"He is older than her" is unacceptable English; by implication it means "He is older than her is."
I have already written to dispute this and been accepted. According to Cambridge's English Grammar in Use, BOTH are correct. You use the object "He is older than HER" when there is no second verb, but if you use the verb you must use the subject "He is older than SHE is". As for the "grande" part, in this case it refers to age, older.
Interesting. Thank you for enlightening me. I have also recently learned that "none" can take BOTH "are" AND "is." :)
Since when does grande mean older. According to DL, my answer was marked wrong because I said "... bigger than she", and they wanted "she's" - that would not be correct in English; one would either say "... than she is" or just omit the final "is", which is implied or understood.