Some research shows that children as young as 4 years old can identify a major problem exists. However, most children this young will not offer help. Some children this age will seek an adult and inform them.
It depends on the environment the child is in, their education level and what they have been taught at home and in their community. In South Africa, children as young as 5 may phone a helpline, but this is because they are taught lifeskills at school and what to do when something is wrong. Children are able to identify a problem, if they have been taught what is 'wrong' or 'right' Under the age of 7 they do not have metacognitive functions and thus may not have the framework with which to process that something is a problem, however if they have simple knowledge about what is not right they are able to categorise 'wrong' and 'right' behaviours, such as physical/sexual abuse. It is feasible that a child of 4 can identify a problem and ask for help based on the above.
Sorry, if you are talking about medical lifesaving skills, I can say that Lifeguards and nippers and pre nippers actually know CPR here in South Africa, as they are trained to apply first aid and learn it as well through watching other lifeguards train. They are also taught emergency numbers.