Children in this stage lack conceptions of size, volume, and mass so teaching a child about something like portion sizes of food in a discussion of nutrition would be ineffectual at this stage.
Concrete Operational Stage (ages 7-11)
During this stage, children can understand the concept of multiple stages or aspects of a problem, the concept of transferable size and volume, and also reversibility of things like numbers or steps of an action. A child at this stage can understand, for example, that a large plate of fries and a small plate of fries have the same amount of food, even though the portion looks smaller on the large plate. Also, the child at this stage is no longer egocentric. The child can understand that he or she must undergo a difficult treatment, even though it hurts, because the family wants the child to get well, or that he or she may need to do things differently than others in his or her peer group. For example, a child who is a diabetic can understand that he or she cannot eat the same candy as his or her peers and suffer no adverse consequences, even though most of the peer group does not need to check their blood sugar. Children are still operating in the realm of the concrete at this stage, however, so rather than stress the moral rightness or wrongness of actions on an abstract level it is better to focus on physical consequences and results.
Formal Operational State (ages 11+)
After puberty, during the formal operational state, children begin to acquire the ability to think on an abstract level. A teenage affected with cancer might have questions to the philosophical question of 'why me,' or a teenager contemplating using birth control can weigh the pros and cons or long-term consequences of engaging in sexual behavior, or at least the different side effects of different methods of birth control. At this stage, children are no longer functioning on a purely egocentric and literal level, and can understand that he or she may have different perceptions and feelings about different issues than his or her parents, for example.
Piaget believed that the final transition into the formal operational stage usually coincided with puberty, although not all individuals made a full transition into the ability to think rationally, logically, and on an abstract level. It is up to the nurse's discretion when dealing with teenagers to judge, based upon the individual, whether the patient is interested or capable of formal operational logic. Also, all of the transitions to the different phases happen at different ages for different persons.