Babysitters typically look after children while parents go out for an evening or afternoon. There is no legal minimum age at which children may be left alone, but parents can be prosecuted for neglect if it puts them at risk of injury or suffering.
The NSPCC advises babysitters should be older than 16 years. If you use a babysitter under 16 and the child is injured in their care, you may be held responsible. So choose babysitters carefully and ask for references if you do not know the person well.
The NSPCC have produced a helpful leaflet Home Alone - is my child ready to be left alone?