ETA relevant links: https://youtube.com/@NotJustBikes https://letgrow.org/
ETA again: I glibly mentioned "being killed by a driver" but of course navigating the typical US built environment if you're under 16 or otherwise unable to drive is a miserable experience in a number of ways even if you survive it. Highways make pedestrian paths unnecessarily roundabout. Parking lots make everything further from everything else. Crossing major roads requires getting drivers to notice and stop for you (harder when you're short!), or waiting through interminable signal cycles, etc.
I'm more than happy to transition into a secondary caregiver for their own children and/or a financial backstop when they get to that stage of life. But I've met too many people first hand who were never allowed to increase their own autonomy as they aged into adulthood. Sometimes due to their parents and sometimes due to society. They do not seem to be happy.