I don’t know if I fully understand all of your questions but I will try to answer them, in the order asked.
People with schizophrenia, may hear voices and may see things that are not there. Some of these “visions” may be associated with past trauma. Individuals who reexperience traumatic events may be experiencing flashbacks. They are associated with post-traumatic stress disorder (PTSD). It would be advantageous to be evaluated by a mental health professional who can investigate whether the “visions” are associated with PTSD or schizophrenia. It is possible that the visions are associated with both disorders.
The exacerbation of hallucinations can be influenced by highly stressful events but I’m not certain what you mean by “coexisting with other schizophrenic people.” Having schizophrenia and being in the presence of individuals with schizophrenia may be stressful if they are actively psychotic but it would depend on the circumstances.
A person with schizophrenia may feel as though they are being constantly watched (i.e. paranoia) but this is not the case with all individuals with schizophrenia. Symptoms vary, person to person.
The majority of individuals who develop schizophrenia do so between the ages of 18 and 25. It is also possible to develop schizophrenia after the age of 30. Generally, an individual who develops schizophrenia has shown some earlier signs of the disorder, but that is not true in every case. No one is certain what causes schizophrenia but the development of the disorder may be associated with a significant trauma.
Research has shown that schizophrenia may be hereditary. Children who have a parent with schizophrenia have a 13- to 15 percent increased risk for developing the disorder. Just because a parent has schizophrenia does not mean their offspring will develop it. Whether someone develops schizophrenia is determined by many factors.
I don’t fully understand your final question but an individual with schizophrenia may believe that other people are attempting to interfere with them. For instance, an individual may believe that someone is inserting thoughts into their brain, following them, or otherwise attempting to harm them.
I hope that I have adequately answered your questions. If you have additional questions or need clarification about my answers, please don’t hesitate to write again.