At what age do you believe it is acceptable to leave a child home alone?

Assume average child, no autism, no special needs.

When is it OK to let the kid be home alone…

…to get himself off to school via bus?
...to walk or bike to school?
...while parent(s) make a quick run to the grocery?
...while the parent(s) go on a date?

Does it make a difference if the kid is home alone or if there are younger siblings to take care of?

What do you think?

Inspired by @ibstubro’s question regarding the Hawaiian elementary school student’s father, who was sentenced to probation for making his (age unknown) child walk a mile or so home from school