I didn't think there was a legal age for babysitting, it is the responsibility of the parents to assess whether a person is responsible enough to look after their children (and they have to remember that if they leave their children with someone under the age of 16, they can still be prosecuted if anything happens).
Personally I think it totally depends on the person. You can get very mature, sensible 14 year olds, and immature, incapable 18 year olds. Would say no younger than 14 though.