The problem with that is that it led him to put the stupid-ass braces of making ten year old Liz talk funny without taking into consideration that she was sensitive about her appearance and how others would view her because, well, chee, he still saw her as a five year old who wouldn't take that into consideration. John's need to find an alcove where his fragile ego doesn't have to be challenged ever because he's too squishy and weak to deal with conflict like a real man means that he's always at least half a decade behind everyone else.
The reason that I mention this is that he thus becomes sort of an avatar for a mental peculiarity of Lynn herself. Said irritating problem is her tendency to not let a boring thing like continuity get in the way of a 'good' punchline. If a joke requires John and Elly to forget that she has a day job on top of housework, Elly isn't used to being in the labor force but if we require a sounding board for her antipathy towards the jock culture that's clearly a threat because she herself wants to boil her gym teacher in oil for making her experience actual pain, she's the quiet power behind Story Time. John's inability to remember what people are up to is thus an expression of Lynn's lack of concern as to where people are in their lives if that should get in the way of sophomoric humor and stereotyping.