My bro was born 14 years before me and he was NOT smarter than me.
My older brother thinks he’s smarter than me.
IMO, first born usually receive more adult personal attention than does a second (or additional) children and typically mature a bit faster, often diagnosed as being “smarter”. I didn’t need some government paid study to tell me that.
Of course the relationship is not deterministic.
It only takes a small statistical advantage to have a large aggregate effect.
My sister is 4 years younger than me and thinks calculus, algebra, trig, and the like is fun and simple.
I barely passed algebra.
On the other hand, I was good at reasoning long winded accounting problems. My sister would go into meltdowns.
Its a general trend that is observable, but like everything it is not 100% true.
Generally this I believe is nothing more than the simple fact that a first born child receives more attention in general from their parents during the years prior to other siblings births, and with each passing child, the amount of time and attention available for each child lessens.
Its a nurture thing, not a nature thing.