Depends on the child (and the parents ...)!
The now adult triplet kids of friends is a case in point: girl was stereotypically ditzy blonde who still needed to be with someone in order to get out at the correct exit when she was 11; one boy had all his ducks in a row by the time he could climb the steps unaided (including being able to differentiate between people in "real" uniforms and other strangers) - could safely be sent off to his music lessons alone - a bus & a train ride - by the time he was 8; other boy with absent-minded professor gene who knew all the bus routes, station layouts and ferry timetables, but to this day - in his late 20s - still occasionally finds himself on the Mui Wo ferry or 5X to North Point when he set out for Shatin. At least in these days of mobile phones his wife can find out where he is!