In the 1970s, everyone thought it was reasonable for me to start taking the subway to school alone when I was 11: my parents, my friends' parents, the school, the subway token booth clerks who looked at my transit pass...
Yes, there's selection bias there: kids whose parents didn't think that was a reasonable idea stayed in neighborhood schools. But the social infrastructure supported it: if my parents were OK with me taking the subway alone to school or the dentist's office, so were the school and the dentist's office and so on. We didn't worry that some well-meaning stranger would make a fuss.
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Yes, there's selection bias there: kids whose parents didn't think that was a reasonable idea stayed in neighborhood schools. But the social infrastructure supported it: if my parents were OK with me taking the subway alone to school or the dentist's office, so were the school and the dentist's office and so on. We didn't worry that some well-meaning stranger would make a fuss.