It's the age cohort thing. A woman has about twenty years of reproductive viability, there's on average twenty years between birth and when the kid is old enough to marry and make a home to have kids grow up in, so at most that's the age range for a shared generational experience.

It doesn't work that way necessarily, and of course we get middle age males divorcing, rolling the dice with a woman his age or younger, but by forty you are slowing down and too old for young kids, plus you will be retired by the time they reach college and/or adulthood. The age cohort breaks that twenty-some years into mini cohorts, 2-4 of 5-11 years and the 'Disney vault' is based on seven. Most families try for at least two year spacing, sometimes four or five so the 2-3 kids in a family tend to be all the same mini age cohort, mostly, and possibly their cousins, especially with smaller families. Two sets of two with a small spread, or three sets of three with up to twenty years