Sorry damsonjam I am not trying to get at you by disagreeing with what you say, but I think LAs will have different rules about babysitters for LA children. It would depend on the maturity of the person concerned although I agree 18 would seem to be the nor (as they are no longer a minor), with as you say a full CRB check.
I have always been under the impression that legally there is no actual age that is legal for a baby sitter for a birth child, be they family member or related. Apologies if I am wrong but would be interested in the source of your post re the age of baby sitters for birth children.
It has always been my understanding that there is no legal age but if a parent leaves a child with a babysitter who proves to be incapable by virtue of the fact that eg an accident happens that compromises the health and safety of the children, then the parents can be charged with being negligent.
I accept that I may be wrong so would be interested in your source re ages of babysitters for birth children.
I wasn't actually meaning birth children just foster children. although for what it's worth there is no minimum legal age that you can leave your children. So in theory you can leave your 4 year old home alone, until they have an accident and then you will be prosecuted !
So far all my babysitters have been 30 plus but things will change when I start fostering as usually my ds goes to their house which won't be allowed for the age of fc I will be having. I really hope my 18 year old friend will be approved as she is very mature and reliable.