I know there is no law and it depends on the maturity of the child. But my ds is nearly 14 and looks after dd (6) for short periods during the day time (up to 20 minutes). But i've never gone out for an evening and left them yet.
I think he would be absolutely fine but I sort of want to wait til he's 14, it feels more responsible of me.
What age are other people leaving their older kids to babysit younger ones?