I grew up in a town of 500,000 people, although we lived in a residential neighbourhood away from the city (16 blocks by 16 blocks). I walked to school alone starting in Kindergarten (5 blocks, crossing busy street with crossing guard). My siblings and I were allowed to roam within our neighbourhood from the time we were 8, without supervision. By the time we were 13, we were allowed to roam into the city (about 5km away). We had strict time schedules at first, but at 14, we just had to say where we were going and the rough time we were coming back. It wasn't safer back then (the year I was born, they caught a serial child killer who had been plaguing our city for 20 years), but my parents taught us how to take care of ourselves alone.
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I now live in a town of 150,000 people and we live in a residential neighbourhood away from the centre of town. My kids (8 and 11) were allowed to play unattended on our street (cul-de-sac) from 5 years old, go to other streets at 7 years old, and bike to school alone at 10 years old (the older one accompanies the younger one and the bike paths follow busy streets). Both of them have been allowed to roam our neighbourhood, but unless they're going in a big group, they have strict time limits. My 11 year old is allowed to bike to other neighbourhoods to visit friends, but neither of them may bike to town without me (it's much much too busy with too much traffic, and too many roundabouts). It depends on the situation, but I think I"ll allow them to do that when they're 16.
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I think it depends on the child, but I also think that children should be allowed to roam and do their own thing. We as parents should teach them how to go about this safely. I personally believe that the world isn't any more dangerous than before and we can't put our kids in a bubble just because we're afraid (especially if the fears are unfounded).