Too many variables to say.
Where I grew up, in the 1970's, in an American inner city neighbourhood with an extremely high crime rate (ie, I regularly heard gunshots at night), it was not safe to go out. In fact, I spent SO much time sitting on my rear in our small apartment, that I developed a significant swayback and had to get physiotherapy when I was six.
But when I was nine, we moved to Canada and my mother thought she'd hit the promised land. Sure, we were still in a major metropolitan area, but compared to where we came from it looked safe as houses to her, so she was all, "Run! Be free!" and I was, "Wait... what's the difference between left and right again?" I got lost SO many times in our city. But, I always found my way home again, eventually!
When I had kids of my own, in the 1990's, we were living in a quiet neighbourhood. Amenities all within walking distance. Few cars on the side streets. People would leave their doors unlocked all day. So yep... my kids got that quintessential childhood experience of being turned out doors and come dinner time, I'm going from yard to yard looking for them.
I really don't think it's a "different world" now. I still see packs of kids roaming freely in my neighbourhood. I tutor 11 year olds who walk several kilometers to get home, and who take public transit on their own. One of my seven year old students walks a couple blocks home on his own, on days when I have another student right after him. He thinks I'm over-protective, because I walk with him on the other days, but it's really a ruse to get him to practice his conversational skills with me (I get him to play rhyming "games" and such, while we walk).
I WAS horrified when I saw a mum let her two year old outside to play street hockey alone with the bigger boys... but that was because MY 2yo was still trying to put broken glass in her mouth, eat drywall, chow down on bugs, etc. Her child was probably just more sensible than mine. (And, to be fair, the boys were keeping an eye on the little guy.)
Anyway... I think with cellphones, I'd probably be okay with letting my kids run off alone in the parks together at 10 and 12. For sure, we did it at Universal when they were 12 and 14 (and they didn't even have a cellphone, that time!).