Schizophrenia is not a formless illness. It doesn't come and go in a random pattern. There is a definitive onset and parameters of onset that have been studied extensively, allowing clinicians as well as the general population to get some kind of idea about the nature of this illness and what can be expected.
The norm for people to first begin experiencing the symptoms of schizophrenia is late adolescence, typically between the ages of 18 and 21, but it can range anywhere between 16 and 45, with very few people experiencing the onset of schizophrenia past the age of 45.
It is very unusual to find people experiencing schizophrenia prior to the age of 18 and younger than that is even more rare.
The reasons why late adolescence seems to be the age of onset are not entirely clear and are still being studied. However, there is evidence pointing to the sensitive period in brain development which coincides with the onset of schizophrenia between the ages of 18 and 21.
Others point to the high level of stress accompanying one's entry into the adult world; leaving home and family; embarking on a job or independence in living arrangements, college and so forth. The "nature versus nurture" argument with regards to schizophrenia is subtle and ongoing. There seems to be a genetic predisposition to developing schizophrenia with environmental stressors sometimes triggering the onset of schizophrenia.
Although schizophrenia does run in families, one is not automatically at risk of developing schizophrenia simply because there are others with it in the family.
Scientists are still studying the correlation between schizophrenia and environmental, social, nutritional and emotional components. There is much to be learned and much to be thankful for as we realize how far we have come in treating, understanding and living with this illness.
'Early Onset Schizophrenia', National Alliance on Mental Illness http://www.nami.org/Content/ContentGroups/Helpline1/Early_Onset_Schizophrenia.htm
National Institute of Mental Health: 'What Is Schizophrenia?' http://www.nimh.nih.gov/health/publications/schizophrenia/complete-index.shtml