What Age to Walk Alone?

What age do you think it’s okay for a kid to walk to the bus stop alone?

This morning I was faced with a situation where Hubby had to leave early for work so he wasn’t able to be here to watch the other 2 Monsters while I took Monkey to the bus stop. I didn’t want to bring the other two because it was freezing outside and I don’t have an appropriate bunting bag/coat/snowsuit for BabyGirl yet. I told Monkey we were going to be really quick because I had to leave the other two here alone and Monkey who is 4.5 years old said “I can walk by myself Mom”. He was very confident and sure. It’s only 3 doors down, but I didn’t want to let him walk alone. In the end I decided to bring the baby monitor and left BabyGirl in her crib with a few toys and LoveBug watching TV. What would you have done? How old does a kid need to be to get to the bus stop alone?