Yes - it is most often manifested in late adolescent or young adulthood. The reasoning for this is not commonly understood. Still others think that by late adolescence the brain is fully matured and there is some trigger or hormonal situation that sets it off. But usually there are some prodromal signs for several years before an actual psychotic break. Once a person develops schizophrenia you often hear them say that they changed - became more withdrawn over the last several years and seemed different.
Stress is also one of the reasons given for when the actual psychotic episode occurs.
There are few specific tests to diagnose schizophrenia. However, people are getting PET scans of the brain and other neuro imaging. Mostly it is diagnosed by history and the length, severity, and type of symptoms. The other thing that is so sad about this illness is that there is no known prevention. CAT scan of the head may show enlarged ventricles - bit that is not always the case.