Age gaps mean nothing, I have 4 younger siblings
DB 1= 18 months age gap and we never really got along (maybe the first 2-3 years of our life were about fine, and then we started despising each other and still very much do almost 3 decades later, as we are the total opposite of each other.) We are now both adults and no contact.
DB2= 6 years gap, and I call him my favorite sibling (a close call with DB3) as he is the most similar in personality. Honestly I can’t recall how close we were as kids, I think he was by default closer to my brother (a 4 year gap) but we always got along just fine as he is a very easy going person who just gets along with everyone and go with the flow. We live in different country and have different lives but are as close as we can about be under the circumstances.
DB3= 12 years gap. He is the sibling with whom I actually have the most memories as I was older when he was born and liked to look after him, due to the gap, we definitely weren’t close “friends” as kids as completely different lives but were close and are still very close now. He is actually the sibling I see most and usually spoilt and see the most and do think I will maintain the closeness as years keep going by (he is a late teenager).
Dsis 4= 25 years gap, I was well into adulthood when she was born and she is still obviously very young so completely different bond to the other siblings with whom I, at some point, shared a roof with, but we are decently close and I treat her more like a little niece than a sister and due to the age gap I do tend to spoil her and she does tend to look up to me. That being said we live in completely different countries and have completely different lives so our contacts are limited to a few times a year.
As you can see, it’s not the age gap that define the closeness; it’s very much personality. DB2 could have been born any year and I am pretty sure we would be close as he is someone I truly enjoy and appreciate as a human (irrelevant of family links) DB1 on the other hand could have been born 6, 12, or 25 years later and I probably would still be no or low contact as he is someone I simply don’t enjoy and really can’t relate to/find massively toxic.