Surely, this must depend on the child and if any other younger siblings are involved. Some children mature quickly and some a lot slower. Each parent must be the one to make a choice based on knowing their child through and through.
In parenting, there are far too many arbitrary rules imposed on us by various governments and other people, telling us what we must or must not do when really, most parents are able to make their own choices without all this 'guidance'.
On that note, I issue a royal decree that no child must be left on their own for thirty minutes without reaching the grand old age of 10!