Are older women threatened by younger women?

My girlfriend is 27 years old and occasionally we will go to a hardware store to pick up items. There's this very pretty girl that works there and she's very nice and sweet. Usually when I go I bring my younger brother because he has a crush on her, but sometimes I bring my wife and she treats this girl really bad and gets an attitude with her. She says she doesn't like that girl even though they look like they can be sisters. Just the other day I brought her there and she snapped on the girl for looking at her funny. She actually confronted the girl about it. Most of the time the girl remains polite, but this time she started taking shots back so I sat there and defused the situation. My wife isn't usually like this. Is it a girl thing or something else?
Are older women threatened by younger women?
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