When Ann had been married to someone (Ed) who was so much older than she was?

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While it may appear hypocritical on the surface, Ann Romano’s reaction was rooted more in trauma and regret than a double standard.

In the three-part episode "The Older Man" (Season 2, Episodes 14–16), Ann freaks out when 17-year-old Julie begins dating (and eventually receives a marriage proposal from) Paul Hanley, a 42-year-old man.
Here is why her reaction was consistent with her character arc rather than just simple hypocrisy:

Mirroring Her Own "Mistake"
Ann didn't just marry young; she married Ed Cooper at 17 because she was pregnant with Julie. She spent the entire series viewing that decision as a "cage" that robbed her of her youth and independence. Seeing Julie enter a relationship with a 42-year-old—the same age Ann was during later seasons—was a literal manifestation of her fear that Julie would trade her own future for a man's life.

The Age Gap Difference
In the 1970s, it was often implied that Ed was several years older than Ann, but the show never established him as being 25 years older, which was the gap between Julie (17) and Paul (42). This massive gap suggested a power imbalance that deeply unsettled Ann.

Power Dynamics
Ann’s struggle was for her daughters to have the choices she never had. She believed a 42-year-old man would naturally dominate a teenager's life, preventing Julie from ever discovering who she was on her own—which was the central theme of Ann's own journey.

The "Ed" Factor
Interestingly, when Julie's father, Ed, arrives to stop the marriage, Julie points out that their marriage didn't fail because of an age difference, but because they were "wrong" for each other. Ann eventually softened her stance, realizing that pushing Julie away would only lose her her daughter, and famously decided it was better to "gain an old son than lose a young daughter".

Ultimately, Ann wasn't saying "don't do what I did"; she was saying "don't let your life be decided for you at 17," which was the one mistake she regretted most.