Hmm, I have to say I never got the sense that Ginny and Harry were meant to be replaying Lily and James--and if anything, I feel like we knew way more about Ginny in the end than we did about Lily, whose importance ultimately came down to the men in her life (oh JKR why).

I think that wizards/witches marry quite young, and generally are continuing their educations or apprenticeships during the first 11 or so years of child-raising. Then, when the kids leave the nest, the wizard/wtich's career *really* starts.

I don't think that's very cracky at all! Considering that 80 seems to be about the baseline lifespan for even a mediocre witch or wizard (and Filch, who's a Squib, is pretty spry), it would make sense that people wouldn't really start coming into their mature abilities until 30s and 40s.