Something that came up in a conversation offsite between me and Adelene Dawner:
Both in canon and MoR, where are all the grandparents and great-grandparents?
Supposedly, wizards have much longer lifespans than Muggles. I'm a Muggle, about to turn 22, and I've still got a grandparent left. Meanwhile, baby Harry managed to be orphaned without any of his grandparents stepping forward to take him in, or even trying to have a relationship with him. Perhaps Lily and Petunia's folks, Muggles both, were dead by this time - they never show up in canon - but what happened to pureblood James's mom and dad? Or their parents, or their siblings - when these people could all easily have lived to be a hundred years old, there should be some many-generation families running around.
The only visible ancestors we have before the canon epilogue are Augusta Longbottom, and, by the end of the series, Andromeda Tonks. Old characters like Dumbledore and McGonagall exist, but seem unmarried, childless, grandchildless. The Weasleys had at least one great-aunt and one great-uncle, but neither Molly nor Arthur has parents coming around for dinner, and they try to be an awfully close-knit family.
They have just been through a war - many such people may have died. It's also possible that most wizards/witches had children much later, and the relatively 'young' families we see were a response to the war.
[Update: and now there's a fifth discussion thread, which you should probably use in preference to this one. Later update: and a sixth -- in the discussion section, which is where these threads are living for now on. Also: tag for HP threads in the main section, and tag for HP threads in the discussion section.]
The third discussion thread is above 500 comments now, just like the others, so it's time for a new one. Predecessors: one, two, three. For anyone who's been on Mars and doesn't know what this is about: it's Eliezer's remarkable Harry Potter fanfic.
Spoiler warning and helpful suggestion (copied from those in the earlier threads):
Spoiler Warning: this thread contains unrot13'd spoilers for Harry Potter and the Methods of Rationality up to the current chapter and for the original Harry Potter series. Please continue to use rot13 for spoilers to other works of fiction, or if you have insider knowledge of future chapters of Harry Potter and the Methods of Rationality.
A suggestion: mention at the top of your comment which chapter you're commenting on, or what chapter you're up to, so that people can understand the context of your comment even after more chapters have been posted. This can also help people avoid reading spoilers for a new chapter before they realize that there is a new chapter.