Schizophrenia is contributed to by more than 120 genes. However having a genetic predisposition, or having a relative with schizophrenia, doesn't necessarily mean that you will develop Schizophrenia.
Late adolescence and early adulthood is the time where it most commonly manifests. Psychological and social pressures can precipitate the illness or make it worse. It is important to look at Schizophrenia as being an interaction between the person and his/her environment.