Why is it considered okay for a 40 year old to date an 18 year old, but it's "gross" when an 18 year old dates a 16/17 year old?

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Am I the only one who finds that shit weird?
Why is it considered okay for a 40 year old to date an 18 year old, but it's "gross" when an 18 year old dates a 16/17 year old?
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