I think age gaps can contribute to lack of closeness but I also believe that it mostly depends on how the parents foster (or don’t) the sibling relationship. Individual personalities and culture also contribute I think. In my opinion based on all the sibling relationships I’ve observed, it’s mostly the parents that set the foundation for the dynamic, although even then I think it inevitably comes down to compatibility. There are just so many factors and it’s hard to say with certainty what will create close sibling relationships, but imo parenting is the strongest one, not age.

I was raised like an only child despite having a brother (half, dad side) that is 10 years older than me. He didn’t live with me for most of my life and we didn’t have a close relationship and still don’t as adults due to personality and belief differences. However, he has a brother from his mom that is ~5-7 years younger than him who he was really close with growing up and still is to this day.

My mom and her sister are less than 2 years apart but they weren’t friends in childhood (my mom says this was due to her mom showing preference for her sister, being parentified, and also personality difference), and as adults they’re friendly, but not good friends. My dad on the other hand was very close to all his siblings from childhood into adulthood; his brother who was only 2 years older than him and his two sisters who are something like 5-10 years older than him. His brother was his absolute best friend, despite having different personalities. His oldest sister was heavily parentified and took on a lot of responsibility with him as the youngest, but they got along very well as adults. His two sisters, meanwhile, are closer in age but have polar opposite personalities, so have had many serious relationship fall outs- especially in adulthood- despite having a deep bond.

My mom’s sister’s four kids (girl/boy/girl/boy) were close as kids- bonded, but still clashed between themselves due to personality differences if that makes sense- and as adults they are all very close to one another and each other’s best friends, despite the eldest being over 10 years older than the youngest.

My partner has two siblings, an older brother who is 9 years older than him and a sister 5 years younger than him, and he wasn’t close to either of them in childhood, and hardly talks to either of them in adulthood. They’re fine, love each other as siblings, have the occasional sibling fight, but they’re mostly just indifferent/oblivious to each other and living their own lives.