First, as your question implies, developmental level must be taken into consideration. After about 6-7 years of age, however, these events should decrease in frequency.
Schizophrenia seems to be very rare before the age of six.
Delusions and hallucinations in school age children are usually associated with serious medical or psychiatric disorders. The frequency of schizophrenia increases after about the age of eleven, and reaches its maximum in late adolescence or early adulthood. To complicate matters, schizophrenia may exist in association with learning disabilities, mental retardation, conduct disorders and autism. A thorough developmental history, psychiatric examination, psychological testing and medical evaluation are necessary for diagnosis. Of course, some of these may be present in some forms of mental retardation and a variety of neurologic disorders.