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10
There is no rule. My 91-year-old father inlaw still refers to me as one of the kids. As hard as it might be for you to grasp, there doesn't have to be a rule about everything.
probably when they act like one
Many people never grow up mentally even though they reach a very advance physical age.
If I'm 70 and you're 40, you're a kid. If I'm demented at 70, I'm a kid. If I'm 30 and you're 45 I'm a kid.
It's all relative.