In: Psychology
Hello thank you for the question,
The answer to it would be that mental disorders are caused as a result of both Genetic ie biological factors and Environmental factors. You cannot separate them from each other. Whether genes caused a disorder or whether it is the environment is hard to tell and research hasn't been able to figure that out yet.
To explain this, supposed person X has a family history of depression, therefore he will be more likely to get depression himself ie he processes the gene of depression. But if he has a loving family or is brought up in an environment that is supportive the gene for depression may never get activated and therefore person X may never get any mental disorders.
Environmental factors are the stressors that can trigger the manifestation of a disorder and what causes stress for one person, may differ from another. They are subjective and can be anything from divorce, abuse, trauma, exams, unemployment, breakup, deaths etc
This is also the reason that even though siblings may have the same genes and are brought up in the same environment both may not suffer from a mental disorder.
Protective factors are elements that help an individual deal with stressors in the environment. If a person is able to cope effectively with his environment he may never suffer from any disorder. They are the individual's personality, his positive attitude, the social support that he has etc.
To answer your question as to why adults experience mental illness in adulthood, it need not always be true. There is a separate category for childhood disorders in the DSM 5 that includes ADHD, Autism etc The manifestation of a disorder depends on environmental stressors. Children mostly do not have as many stressors as adults. Their brain is not yet developed fully and they do not understand issues on a deeper level like adults. Their thinking capacity is limited. However, it can affect them in the long run.
You do, however, find many pre symptoms of a disorder prevalent in childhood like for eg in schizophrenia many schizophrenics as children had reported to be very reserved, did not show any emotional responses and did not engage in social activities.
Schizophrenia, Depression, Substance abuse-related disorders, Eating disorders are more likely to be present in adulthood.
Hope this answered your question! :)