system
Given the following post, respond as if you're commenting on Reddit:
human
What is the Neurological reason why Schizophrenia is almost always diagnosed in people between the ages of 18-25 ish?

Also, why are Smoking Marijuana in Teenage Years, Living in an Urban Area Risk, and Social Isolation all Major Risk Factors? 

What else can people do to reduce their risk of developing this disorder if they are genetically prone to it?
gpt
The reason schizophrenia is almost always diagnosed late is because the "big " symptoms - or what we call "positive symptoms" like crisis, delusion/hallucination etc - start to appear during adolescence/young adulthood. When the positive symptoms appear, and if they are chronic or persist, there is less doubt on the diagnosis. 

On the other hand, years before the positive symptoms appear, there are "negative symptoms" that start to be visible - which can be autistic/depressive like and they can go unnoticed/underestimated/misdiagnosed. hopefully we will develop new tools to spot or predict schizophrenia ahead of the positive symptoms appearance.  

As far as what is the risk/how to protect yourself, little is known yet - our best bet is to say that it must be a specific combination of environmental + genetical factors that we don't fully understand (and not purely genetical nor evironmental). 

There are some good reasons to think that schizphrenia is a neurodevelopmental disorder - if you look at it from this perspective - some repeated interactions, like drug abuse, high stress, etc during childhood can affect the brain and how it will develop. So that could be an explanation as to why some variables seem to 'increase' the risk of developing schizophrenia.