Thankfully, Duo also accepts "She's an elderly person" - common in the UK, at least. An American will have to confirm if "elderly" is used in the USA too.
"She is elderly" is an even shorter possible (untested) translation, though perhaps too inexact for Duo to accept.
she is old also works and a suggested answer 'she is an old man' is a bit ridiculous, no?
It probably was deleted after bloogy182's comment. It would be an a bit ridiculous sentence indeed.
And what about "She is an old woman"? If you say that "She is an old man" would be a bit ridiculous sentence indeed.
Czy kobieta nie jest człowiekiem?
this is complicated issue with both "man" and "człowiek" being complicated words.
"woman" and "kobieta" are almost 100% 1 to 1 translation.
But in English in some sentences "man" means "human being" and in some sentences means "male human being"
And while "człowiek" generally means "human being", it often means "this male human being".
But you know, some men think "kobieta nie jest człowiekiem" so you might want to clarify.