She could have, you know, TALKED to some actual middle-aged women for starters instead of pulling those ridiculous conclusions out of, uh, thin air. (And yes, she did talk to one author of a book about over-50 beauty, but we don't know that woman's age either, for that matter.)
And she could definitely have not written in a voice that strongly implies that she is a member of the age group about which she writes. (E.g. "The rest of us need to invest in [Spanx, etc.].")
If a slender person wrote about diet tips, or a rich person wrote about poverty, and they demonstrated such colossal ignorance and stereotyping, I'd have the same reaction.