When I was in my mid 20s I noticed I was suddenly interacting much more with my sister Cindy who is 6 years younger, but it was unclear as to why. Did she suddenly become smarter or was there more to it? After a lot of thought, I arrived at the “4 year rule” – that every 4 years our personas go thru a metamorphosis. As example, 3 and 7 year olds have nothing in common, nor 7 vs. 11, 11 vs. 15, even 15 vs. 19. The pace of personal development is so earth-shattering frankly. But then we get to our 20s and the pace eases up, a lot. Hence why I only discovered my sisters so late.
The implication is that an age gap of greater than 4 years means you will not materially “share the ride” with your older or younger sibling. Each child has what is essentially an “only child” and “oldest child” experience, working from zero insitutionalized knowledge regarding what to expect.
Now when I see young parents with say, a 2 year old, I wonder if they have in mind to have another baby, and if so, when. Wait too long, and the younger one will only have an older sibling they know of, rather then know innately.