Mental illness developing in midlife
I am not a professional, but it is my understanding that it is common for major depression to not show up in adults until the age of 30 to 60 years old; see for example, Table 2 of Lifetime Prevalence and Age-of-Onset Distributions of DSM-IV Disorders in the National Comorbidity Survey Replication, Mood Disorders - Major Depressive Disorder.  Overall, the most likely age group for any disorder to present itself is the years 30-44.

The beginning of the paper states: "Little is known about lifetime prevalence or age of onset of DSM-IV disorders".  The study included 9282 participants.