We live a 5 minute walk (350 metres) to my sons Primary School. There is no road to cross, it's a straight pavement all the way down the hill. By Hong Kong standards where we are located is relatively suburban, so heavy traffic, apart from the school run.
I think my 5.5 year old son is mature enough to start walking to school by himself. Very good with crossing the road. Aware of stranger danger. Who are 'safe' adults to ask for help if he got lost.
But he doesn't want to. He's usually pretty independent with most things. (He's the eldest). He'll happily run down towards the playground by himself, (which is past the school).
I think it's a developmental milestone, like feeding yourself, or getting yourself dressed, etc. None of his friends walk to school, as they are either on school bus, parents drive them in, or they walk with their domestic helpers.
We walked down this morning and I said I would shadow him, walking about 75 meters behind him. He was pretty upset, but continued with coaxing, but arrived at school crying.