Trust me, there is no magical age siblings become friends, whether they are born one, two, three, four, or five years apart. Much comes into play in the outcome of siblings being friends, or not. First, gender is a condition, as is, believe it or not, their zodiac sign. You see, certain zodiac signs are compatible with one another, where as others make for a complete opposite result. I know some people think there is zero correlation between zodiac signs and compatibility, but the reality is, it is true.
I come from a family of six children. My mom gave birth to six children within nine years. So, there is a variety of age groups within a tight 9 years between us six children. Sure enough, those of us who are close to one another do not have anything to do with age, but with our zodiac signs being compatible.
Also, when there is more than a few years in an age difference between siblings, they tend to be starting and finishing up grade school, middle school and high school without their sibling being in the same school but for one year. However, like me and my siblings, once we all reached adulthood, our relationships and our bonds got better because as adults, we all lived the same way. Whereas, as children a few years difference between our ages was a bigger gap because we led different lifestyles within our own circle of friends and where we were in our schooling.
I have two sons. They are two years apart. When they were kids, little kids, they were friends. But, as they aged, they began to butt heads and went their own way with friendships and such. My eldest started school a year late, where as my youngest began early (both entering school when they did, due to the time of year they were born). Therefore they were just one grade year apart in school. My two sons are so different from one another, they could not be any more opposite. Proof positive that kids close in age does not necessarily mean they will be friends.
My little sister is five years younger than I am. When I left home, she was in the 8th grade. We did not have much in common back then. Fast forward to today. As adults, we are closer than we ever have been. I have seen this same scenario in our family and also watched some of my siblings battle one another, still to this day.
Ours, in order of birth are brother, sister, sister, brother, brother, sister.
Brothers 1 and 3 were best buddies when they became adults...8 year age difference
Sisters 1and 2 hated one another growing up. But are best friends today. 1 year apart
Sisters 2 and 3 hardly did anything together as kids, but are best of friends today. 5 years apart
Brother 1 and sister 1 a1ways good friends. 1.5 years apart
Brother 1 and sister 3 only became friends when well into adulthood. 9 years apart
Brother 2 and 3, always fought and were never friends. almost 2 years apart
Brother 1 and 2 always fought and were never friends as kids, but are friends as adults.
Blah, blah, blah.
So, I know for sure, age of siblings do not matter, it is their personality that makes them friends.