Found this on the net:
Q&A: Home alone
With two high profile cases of parents leaving their children to fend for themselves, BBC News Online asked legal expert Katy MacFarlane of the Scottish Child Law Centre to explain the law governing parental responsibility.
Q. Is there a legal age at which children can be left at home alone?
A. No, there is no minimum in either Scotland or England and Wales. The law takes the view that one 13-year-old could be very mature while a 15-year-old equally immature.
Q. Does that mean a parent cannot be prosecuted for leaving their child alone?
A. Not for leaving them alone, per se, but they can be prosecuted if they did not provide enough food, warmth, entertainment or access to emergency help. Or if they were likely to come to harm.
It is also dependant on the length of time the parent is absent. So while half an hour after school might be acceptable, overnight might not.
The 1933 Children and Young Persons Act says it is wilful neglect for a parent to leave a child unsupervised "in a manner likely to cause unnecessary suffering or injury to health".
Q. At what age is it advisable to leave a child alone?
A. It depends on the child. But we would recommend to parents that by 16, a child is being left on his or her own as part of the preparation they need for adulthood. It should be done gradually. The most important thing is that the child is happy