Mental illness developing in midlife There is much research about personality development and childhood. The onset of many mental disorders and illnesses, will occur in early adulthood. Studies are done to link certain childhood personality traits with the manifestation of the illness in adulthood.  Conversely, there is much research into the onset of aged onset dementia, Alzheimer's and the like.

With the exception of the Mid Life Crisis, I have not seen much research on the onset of mental disorders or mental illness in  adults during midlife, or approx. 35-50 years. What studies are there investigating major personality changes later in a person's life?
An example could be the percentage of new diagnoses of "a particular illness" for 35-50 year old age group. 

I am not interested in an ongoing illness that was diagnosed as a younger adult and is  still occurring during this age group. I am interested in the  first time onset of an illness in an individual during these years.
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.