If Duolingo expected "यह आदमी बूढ़ा है।", the English sentence should have been "This man is old". Otherwise, the correct answer should be "यह बूढ़ा है।".
Person could also be व्यक्ति I believe (though perhaps that's more 'individual').
However, यह व्यक्ति बूढ़ी है। was not accepted. Isn't व्यक्ति a feminine noun? Or is there a mistake in some other detail? Thx
At this level of difficulty, I really don't see why it wasn't "This man is old."