And I would cross them over- young Kagyuu meets middle-aged Ren, supposing Ren ages: but for a small problem. As far as I can extrapolate, in Ima's world WW2 didn't happen. This makes for difficulties when trying to map RL history onto manga reality, the primary one being just *when* did Kagyuu grow up and marry?
Assume that the internal time of the series coincides with the date the series began ie mid-90's. Kagyuu has been dead ten years so he dies in the mid-80's. The first Kai story- the only one that gives definite ages to any of Kagyuu's children- happens approximately two years after series start. (I have to look up whether Ritsu was a rounin for one year or two.) In '97 approx Kai was 46, meaning he was born in '51. Kai is eight years older than his youngest brother, meaning Kiyoshi was born in '59 and Kinu was born somewhere between the two. In the family photo Kiyoshi is a baby or young toddler sitting on his mother's lap, making Kai nine or ten, and the photo itself dating from '60 or '61. He has four older sibs. Aya is still in uniform and must be a high school student. We don't know if she or Satoru is the actual oldest, but I'm guessing Satoru is. So make him 18 or so, which means the early children came every two years approximately: Kai 10, Tamaki 12, Kou 14, Aya 16, Satoru 18.
So if Satoru is 18 in 1961, he was born in 1943, which is right in the middle of the war. His parents' courtship and marriage would have happened in the early 40's when the country was already feeling the pinch of the extended campaign in China. Life however seems to go on perfectly normally in the (early 40's) Kagyuu stories to date. Granted that the Makiokas down in Osaka only register the effects of the war at a distance-- well, they're down in a suburb of Osaka. Somehow I'd expect Tokyo to be a little closer to the action and the pinch, even before 1942.
Equally if Kagyuu's first son is born in 1943, and the Kagyuu we see to date is a young writer in his early to mid-20s, then Kagyuu himself was born about 1920. And yeah, the earliest Kagyuu stories look reasonably early Shouwa, no problem. He died in his mid-60s. I have an idea that his wife was possibly a bit younger than he was, just going from looks; and possibly because she seems awfully genki for a 70-something. She *would* have been seventy-something when she taught young Ritsu to drive, two years before series' start. Granted seventy-somethings are often pretty genki (my aunt used to tool about France when she was well into her eighties) still...
There's also the little question of howcome the Iijima house survived the firebombs. I'm all for the house to be in a western suburb, which back before the war was countryside and suburbia. But the original on which Ima based her house was in Mukoujima, on the other side of the Sumida, and that area was a target for the bombs. Of course, that house survived- and has been re-erected elsewhere- so I suppose the Iijima's houses did too. I still prefer to think of it as being somewhere out of Shibuya or Shinjuku, just because.