"Julia is old."
Translation:जूलिया बूढ़ी हैं।
है is fine too. But when you pluralise the noun, by using हैं, it serves as an honorific, to convey respect.
Thats the only option because the default in Indian culture is to use the more respectful case for the elderly. They would like you to internalize this so as to not offend anyone, even if its grammatically fine either way.
There is another question in this lesson that says uses a masculine structure, however, बूढ़ा is used, not बूढ़े. Can anyone clarify?
That sentence forgoes the honorific. You might be of the same age (or older) than the person being referred to in which case you don't use the honorific (unless you want to be formal). It might also be a situation where using the honorific does not make sense (Eg: मेरा कुत्ता बूढ़ा है - My dog is old) or you might be rude/disrespectful intentionally.
I meant in the other sentence that Novakane492 mentioned with बूढ़ा. This particular sentence 'जूलिया बूढ़ी हैं' definitely does use the honorific. Since the feminine form बूढ़ी stays the same when pluralised, you only have the हैं to make that out.