I'm aware the onset of this illness is in the teenage years, but do the symptoms 'decrease' as the patient becomes older – into their 40's and beyond?

Schizophrenia is most likely to developer in later years, from age 30-40 onwards normally. It's unusual to see it in younger years without use of psychoactive substances such as alcohol, nicotine and cannabis, but them symptoms go away when the substance use stops.

Yeah I think so…
NO YOU'RE WRONG!!
I'm sure they decrase with age.
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A clinical study using strict recovery criteria (concurrent remission of positive and negative symptoms and adequate social and vocational functioning continuously for two years) found a recovery rate of 14% within the first five years. a 5-year community study found that 62% showed overall improvement on a composite measure of clinical and functional outcomes