When is too young?

There is a suggestion that a law will be passed making it an offence to leave a young child alone - only question seems to be at what age and for how long?  I'll put my cards on the table - I regularly leave my 9-year-old granddaughter alone in the house while I go to pick up another granddaughter from school - a 10 or so minute trip.  She (the 9-year-old) is a sensible child and knows not to answer the door or answer the 'phone, and although I lock her in, she knows where the front door key is should she need to get out of the house for any reason, and in those circumstances to go round next door.  So I feel I have covered all the bases.  You may ask why I don't take her with me, and the answer is I ask her every time, but she prefers to stay in the house. But it may be that I shall have to rethink this if indeed it becomes illegal to do what I do.  So when is too young, and how long is too long?  Once again we seem to be looking for a "one size fits all" solution to what is definitely not a one size fits all problem.  So much depends on the child - I reckon my 9-year-old could run rings round many kids three and four years older than she.  But she has a schoolfriend of the same age (slightly older in fact) who I most definitely would not trust to be on her own - a completely different character who would probably end up burning the house down or something!  And therein lies the problem with passing laws like this - in order to do good for what is probably a small minority you have to interfere with the lives of the majority who are not in fact those you are seeking to affect.  As somebody-or-other said "God save us from people who mean well"!