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My former English teacher would question why the age of your math teacher is relevant in this post.

Who said anything about the age of their old math teacher? You've got to learn old math and new math, presumably with different teachers.

why are you being ageist? I have a new math teacher, therefore my previous teacher is my old math teacher.

Because you were being pedantic about using fake words they corrected your grammar. Former is more correct than old in this context.