Answered by Irving I. Gottesman, Ph.D.:
The consensus appears to be that schizophrenia is not a degenerative disease, although there are hints that there is some degeneration from autopsy and brain imaging studies. The latter could just be incidental to age.
The appearance of the brain at autopsy (the absence of gliosis) tilts most experts with whom I have talked to favor a neurodevelopmental component which, in concert with genetic predispositions and environmental stressors leads some individuals to develop schizophrenia over a wide age range. Age itself is an independent source of numerous ailments which may add to the worsening of the symptoms of schizophrenia. In an earlier question I answered for this Forum, I also noted that a signficant proportion of patients improve later in life, sometimes without medication, for reasons still unknown.
Periodic medication reviews for type and dosage levels, and even second opinions, are important in the optimal management of all chronic conditions that afflict us. Enough individuals who carry a diagnosis of schizophrenia for many years will then have a manic episode to make it worth noting in the answer to your question; this suggests the presence of bipolar disorder with a necessary switch in medications and a consequent remission. I would also note that one of the consequences of an unrecognized temporal lobe epilepsy is a clinical picture that receives a diagnosis of paranoid schizophrenia; it may take a decade or more for the psychiatric symptoms to appear (see any good textbook of psychiatry).