For the purposes of first aid, what is the defined age of a child?

For purposes of first aid, a child is defined as being between the ages of 1 and about 12 years of age.

For purposes of first aid or CPR, consider infants to be less than one year old and children to be between one and twelve years old. If you cannot determine age well, they are infants until they walk and they are children until they show signs of puberty. You will not be right all the time if you use the walking and puberty rules, but they can help if you do not know exact age.