> Level 0: Blood relatives only. “Uncle”/”Auntie” is strictly for real uncles/aunts (by blood or marriage). No fictive use.
This is definitely not true for children in the USA. I believe it is common (or for sure it was common) that children use Aunt/Uncle for close friends of the family.
It’s sort of an informal Mister/Misses. Kids may call some adult Mister Firstname. But that seems formal for frequent use if the person is around a lot or in a houseguest.
As an example, my best friend from my teens into adulthood is more like a brother to me. His kids are more like nieces and nephews than my own, and more like cousins to my child. They qualify for “sleepover status” well before any of his friends would have slept over, they goof around and have a more deeper relationship with the family, things like that. So it feels natural for my kid to call him Uncle Firstname.
I am in Texas and this does feel like it may be a regional thing fwiw.
EDIT: to add, while this does exist, it's also completely normal and probably more common to just simply call the person Firstname even as a kid. When I think back on when/why it occurs, some people being called Uncle/Aunt, it's seems like the age the kid was introduced to them is relevant. Too old, the kid will likely say Firstname. Young enough, the kid was likely being taught manners and familial titles, so the adult was given one. Some families are more strict regarding these things. Teaching kids, and enforcing compliance, with old school manners like Mr/Mrs seems less and less common.
I remember the usage for friends of the parents in the 1960s
ie ants/Uncles are your parents siblings and cousins are ant/uncle's children so your own age.
But yes I agree boomers don't use it in any case.