I think you can't generalise, because the roads, circumstances depend so much, but
I would expect junior aged children to be able to walk themselves to school quite easily. My ds has done it since he was 7 and prefers to do it. (I walk dds to school, but he goes on ahead and comes home from clubs on his own)
When I was teaching in east End of London, school rule was junior can walk themselves at parents choice, infants must be accompanied. That seems reasonable to me
Obviously if you have very busy road to cross, or walk a long way, that is different.
We live in dead end road, loads of kids in our street and nearby streets, many of them are allowed to play out. My ds has been allowed to walk to friends house 5 minutes away since 8. But again, no road to cross, and he is sensible. dd has been allowed to go and knock for her friend at the end of the road (within sight of our house, but I don't stand and watch) since she was 7.
On the other hand, I would not let older child watch younger child in this context, it isn't fair on older child if something goes wrong.
IMHO we are very overprotective of our kids, and I think that it actually backfires, they don't learn to think and to be street aware. The greatest number of road accidents happen among 11 year olds going to secondary school, because most of them have never been anywhere on their own and are not road wise.