My great grandad brought up in NYC lived in the borough of Queens would take the subway to Manhattan just to walk around and look at stuff. He was eight at the time. No problems, no worries.
My sister hopped a bus and went into NYC when she was 7.
That was years ago, and my mother found out about it recently. She said my sister should have told her where she was going.
Same for me. At 9 my parents let my girlfriend and me take the subway from Queens to Manhattan. No one gave it a second thought. It was a very different world then.