I think it is not only the age gap but personalities.
I have all age gaps.
My first two are 17 mths apart and at 15 and 16 are still close.
Then a 4 yr gap (my 15 and 11 don't get on) the next two are 22 mths apart and get on well (at times )
Then a 6 yr age gap.
The next three have a 24mth age gap between the three (first two 11 mths apart then my last is 13 mth gap)
I like the 18mth age gap the best, I felt it was good for the kids and me, the 11 and 13mth gap are a little to close (now 19ths, 2yrs9mths and 3yrs8mths and a real handful ) but they will go to school one year after each other.
I like that I have three groups close in age so they have someone close to there age.
(I hope all that makes sense )