It depends on the child, the neighborhood, etc., but in general I think that most 8yo's should be just fine playing outside alone.
I've let dd play alone on our deck since she was 3 or so as long as our dogs were out there with her. I was right in the kitchen, which is adjacent to the deck, so it wasn't any different than her being in the living room while I was in the kitchen.
Now that she's 7 (almost 8), I've been struggling to give her more freedom. She is allowed to play out front by herself as long as she tells me that she is going outside. She is allowed to ride her scooter on the sidewalk out front "from Sadie's house to Colette's garage" (this amounts to about a city block, including going around a corner.)
She went to the park alone for the first time the other day. It's not even 1/4 of block (across from Colette's garage actually,) but what made it a big deal was that it was the first that she got to cross a city street by herself. It's not a particularly busy intersection, and there is a four-way stop sign.
For now, that is all that I'm comfortable with letting her do, mostly because of traffic. We live in a residential part of a big city, so there are a lot of cars. Her next big milestone will probably be going to the library alone. It's only about two blocks from our house, and she knows the way like the back of her hand. However, it involves making it across a very busy intersection where three major streets come together at weird angles. It will be a while until I think that she will be ready for that. Actually, she's probably ready, but my nerves aren't. I'd be a wreck waiting for her to come home.