At what age would you, or did you, let your child cross a busy street alone?

Frodo and I were out walking yesterday and saw a woman walking her son on his bike across a busy street. We started chatting and she said she was worried about him crossing alone because of the traffic but wondered at what age you let them do it. I asked how old he was and she said, “Nine.” I started to say that seemed plenty old enough when she said, “I know, borderline.” I may be misremembering but it seems my kids were out and about in the neighborhood on their own by nine. (And I realize it does depend some on the individual kid.) What do y’all think?