A random observation about character age and continuity

I get annoyed - really, really annoyed - with age-related inconsistency and implausibility in books. It annoys me more than any other continuity / suspension of disbelief error would do. (It doesn't annoy me as much as, say, homophobia, racism or bad writing, but on the gnat-bite scale of minor irritants, it ranks high).

By this I mean -

Character A is Character B and C's sibling. Character A is a woman young enough to have a small son, so let's say, what, mid-late 30s? Maybe early 40s? YET CHARACTERS B and C ARE WELL INTO THEIR SIXTIES.

Case in point: Digory's mother and Aunt Letty / Uncle Andrew in The Magician's Nephew.

Or this -

Within the same canon, characters of the same age are described variously as "young", "middle-aged", "elderly", "old", or "in the prime of life".

Agatha Christie, I'm looking at you (and especially the difference in how 40 year old men and 40 year old women are described).

Further, this -

Character A is the parent of Character B and C, but as everyone's age is given at different points in the plot, the application of simple mathematics reveals that Character A was a preternaturally advanced 9-year-old.

(This one pops up oftener than it should in the speculative fiction scene).

I can suspend my disbelief with series characters who don't age at a normal rate - I'm good with people hovering in their 40s or 50s for thirty elapsed book years, that's a step I am willing to take because it's an intentional suspension of natural law for the purposes of story. Laziness with getting age relationships right or possible irritates me, though, and I suspect it always will. It's one of the many reasons that in my own series, I have a file on every recurrent character which includes their birthdates. Call me anal, but my characters ain't ever going to have siblings old enough to be their parents accidentally - if I do that, it'll be on purpose :-)