The only real factor, besides your own potential anxiety, is whether you believe he can take care of himself - including handling problems that may come up. To help decide, I would suggest quizzing him on possible scenarios. Maybe use some role play. For instance -
- You've taken a wrong turn and don't recognize the street, what do you do? (Approach a stranger but stay in public view, ask for directions or to use a phone)
- You run into another kid who wants to take a different route / go play at the park / etc (See if they want to walk with you for company, keep heading to / from school)
- The classic stranger with candy - get in the van, help me find the puppy, etc. (Be polite but stay in public view, keep walking, scream bloody murder if you really feel threatened.)
Suggested answers are just that - suggestions. Use your own judgement - probably on the spot too, because no matter how much you or I think up, your kid will probably come up with their own responses that are very likely good fits to the situation too.
Ultimately, it's one of those "it depends" sort of answers, but I think the role played scenarios should help you figure it out, and will help prepare a kid who isn't quite ready.