In an epidemic, I'd put the cutoff much lower than that - I wouldn't feel entirely comfortable leaving an 8-year old home alone for the whole day in normal cond

In an epidemic, I'd put the cutoff much lower than that - I wouldn't feel entirely comfortable leaving an 8-year old home alone for the whole day in normal conditions, mostly because there's no really good reason for doing this, but during an epidemic that might be the safest option available; if you leave them appropriate lunch then 8 or 9 year old kid should be self-sufficient enough to not do stupid things and phone for assistance if needed; Of course, that doesn't work for younger kids, but then the main focus should be not on schools closing but on preschools and other daycare options closing. The actual practical solution would be the old non-nuclear family; i.e. send the kids (and food supplies) to grandparents and have them spend some time in isolation there, while the "middle generation" works.