At what age do you allow your kids more freedom?

At what age do you give your children more freedom to do things alone?

We live about a 15 minuite walk from the school, you have to walk up a stretch of really busy main road (about 5 mins) and then its a back road (no cars) the rest of the way. Right across from the school there's a park surrounded by fields, a couple of weeks before we broke up for Easter holidays I picked ES (5) up and we were going to go to Morrisons before going home. ES who usually wouldnt stay anywhere alone begged me to let him stay at the park and pick him up on the way home. Now I have no doubt atall that he would NOT have moved from that park until I came back even if i'd been 2 hours but imo he was just to young to leave and it got me thinking what age would I leave him. I don't know.

What age would you allow your kids to go to the park alone, walk home from school alone, go into town alone, that kind of thing?