The NSPCC has laid down guidelines which recommend that the minimum age of being a babysitter should be 16 years. They have based this on the idea that someone over the age of 16 is more likely to be aware of potential dangers and risks and, if necessary, would have the where-withall to deal with an emergency.
There is no age limit at the other end of the age-scale but, as a parent, you should always be aware that the person you are asking to babysit for your child needs to be able to deal with ANY issue which may arise: you need to choose someone with the same abilities and standards which you would expect of yourself. A babysitter needs to be able to make sure that the child is fed, changed, kept safe, given room to play and made to feel secure.
The 16 year age limit recommended by the NSPCC is linked with action which the police could take if anything were to go wrong and an injury resulted, based on the presumption that you as a parent would be responsible if anything goes wrong if your babysitter is under 16 years of age.