Depends on the kid's maturity level. Some kids could deal with that fine at 10, but there are 14 year olds I wouldn't trust to not get into trouble on the way.
I say it depends on the kid and where they live. Where I live, some parents will let their kids go out with friends to the park at 9 or 10, since it's a pretty safe area.
For cities, I say that you should at least be a teenager to go out alone, with a phone to contact somebody if something goes wrong.
I shudder when I think of what was acceptable when I had my kids in the late 1970's. People would walk downtown and go into a store, leaving their sleeping baby in the carriage outside or leave the baby sleeping in the carriage in their yard. My daughter (age 5) walked alone for the last 1/4 mile to Kindergarten class. I could go on...
In terms of being killed, sexually molested or harmed, we'd do much better to watch who we're letting our children spend time with that when and where they're walking around alone. But for stuff like crossing busy roads or walking through neighbourhoods with a lot of gun crime, there's a lot more merit to restricting their movements until they can more fully understand the risks and how to be as safe as possible.