"Ella no es vieja."
Translation:She is not old.
I read in the comments for another answer that "vieja" is usually not for people but for objects while "mayor" is for people but this doesn't seem very right now.
How is it different from "mayor" then?
Shwartz, thanks for this distinction. I have been corrected for this by native-speakers but without telling me why. Now I know. Melita
My Peruvian teacher also told me it was impolite to use viejo/vieja as an adjective to describe people.
Actually, I was referring to the suggested rule because it does not seem to work in the case above. However, it might be that "Mayor" is just a comparative form for "Viejo" when sometimes you would say a person is "Mayor" without comparing to anything because it's just more polite than "viejo". At least, according to this: https://www.duolingo.com/comment/898309
That's because ella does not necessarily refer to a person. La falda no es vieja => Ella no es vieja.
So it can refer both people and objects and maybe usually objects, but still - "she" would not necessarily be correct here but it could be (in case it does refer to a person).