It very much depends on the route and how busy it is, surely.
There's a vast difference between a ten minute walk in a village setting - round the corner, across the village green dotted with local people walking their dogs, then cross one quiet road then turn into your cul-de-sac and you're home.
Or a ten minute walk in town, along pavements full of bustling people, crossing roads with lots of traffic, turn the corner into your estate, past the 'play area' and up the concrete steps to your home, a flat on the 4th floor.
The village setting, 9 or 10 depending on the child and assuming no SEN, as long as they feel confident and ok with the arrangement, and know how to contact you, and what to do in an emergency.
The town setting, 12 or 13, or maybe never? I don't know, though people have to find a way.