I have no idea why this post activated my neurons so badly. Like at first, I disagreed because of the way you worded it, but as I was typing this, I chewed on the post for a little while and realised you make a good point. So, I'm going to give my own thoughts even though I don't tend to do this for posts in general.
From a canonical standpoint, making characters with an age gap that's 10 years or less, siblings rather than parent/child does make sense in almost every case. Simultaneously, though, I feel like sometimes a dynamic just can't be properly explained from a sibling standpoint. Still, you can just argue that they have some different kind of dynamic, like a caretaker dynamic or a cousin dynamic or even just something not easily described.
Typing this has made me come to the conclusion that you're generally right, in my opinion. However, I do somewhat disagree about the particular age difference you set at the end, when you mentioned a minimum of a 20-year age gap.
My reasoning for this is that I have a character that's 11 years older than another character, but he gives me very intense uncle vibes, and I could never in my life imagine him having a sibling dynamic with this girl. He takes the role of a supportive figure, feeds her, gives her advice, makes sure she isn't in danger, etc. The same younger character has a motherly figure that's 16 years older than her. Despite it not being at least a 20-year age difference like you said, I can not for the life of me imagine them not being mother and daughter. A 'sibling' label would do them an extreme disservice.
The younger character has gotten the other mother days gifts and idolises her, they have a mentor/mentee relationship before the motherly character unintentionally adopts her and they have a specific kind of care for eachother that feels a lot more protective in a specific way that I associate with parental care. You could still argue that this doesn't fit, but based on what I know about the characters from personal experience, I do think a mother/daughter relationship fits them well.
A dynamic I have that might prove your point to some extent, however, is a character I have that's around 4 years older than the other. This character takes care of the younger person and acts as her primary caretaker to a certain extent. He offers her shelter and makes sure she's well taken care of. Despite this, they don't have a parent/child relationship. Instead, they have this kind of cousin dynamic that sort of embodies the "one sibling came of age and legally adopted their younger sibling to get them out of a bad house, but they still act like siblings" vibes going on.
Anywho, my point is – I agree that in most cases, a big age gap is necessary for characters to have a viable parent/child dynamic, and that most dynamics with smaller age gaps can be explained differently. I also completely agree with your statement about making characters older. Older characters can have whimsy, too! They can literally be just as fun as younger characters. Make middle-aged guys and sweet old ladies and a 50 year old fantasy traveller or something. You have the freedom to do whatever :D
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