It depends on where you live, first of all. And what your child's temperament is. I don't think any of us can state any particular age. My 8 year old daughter does walk home from the bus stop alone (there are a ton of other kids also walking, and many of their moms meet them there, but I don't...). She lives the closest to the bus stop than any of the kids and despite the child snatching that took place here in Jacksonville a few months ago (resulting in the child's murder), we live in a safe area with lots of other people about. I also allow this child to ride her bike over to her friend's to see if she is home and can play. I would not have done so at this age with my older daughter, probably b/c I was more "protective" then. With my third child I am less worried and more permissive - not in a "bad" way, just, not so freaked out and more encouraging of her to venture out a bit. Mind you, this kid is only going a block away so it's really not a big deal.
 
The mom across the street from us is super protective. Weirdly so, in my opinion. I mean, she plays ball in the driveway with her 8th grade son and has always been outside with her kids when they play, even still. Her son is in 8th grade and is taller than her! Her daughter is 9 or 10 and we live on a tucked-away cul-de-sac for crying out loud. I saw her out there just today. That's not to say she shouldn't play with her kids. I just never see them with friends. She really doesn't let them mix with the neighborhood which is a shame b/c we live in a dream place. Great kids, great families.
 
It's such a personal thing.