What age can kids cross the street alone?

At what age can kids cross the street by themselves? I found myself confronted with this yesterday. Robb was on his way to pick up Kawai from the busstop, but the bus was a few minutes early and had already dropped her off. Luckily a family that lives near the busstop was waiting with her. We live less than a block from the busstop, but you have to cross the street to get to our house, and I'm not ready for her to do that. But what age can she do that?