People with major mental illnesses often do not look forward to a happy and comfortable old age. Long term effects of medication, the illnesses, abuse of street drugs if applicable, and unfortunate life circumstances, often generate bleak or no prospects. I have seen a number of my acquaintances in the mental health treatment system deteriorate when they get older, or die of various causes before getting older.
I have seen people, some of whom I once called friends, take a turn for the worse in terms of personality. Sometimes there is no explanation for this deterioration, and sometimes it seems traceable to a traumatic event.
My wife had a friend who ended the friendship over a head of lettuce. The person had received a great deal of money from someone who had been a perpetrator. The influence of this abuser coming back into this person's life seems to have been more than the person could handle, given the preexisting fragility of having a mental illness. (It would have been better for my wife's past friend to refuse the money-accepting the tainted gift carried too high a spiritual price.)
People with mental illness need to take extra care when there has been a difficult event. The medication can only do so much, especially when a person is overwhelmed with difficult emotions, and may lack the skill to handle these emotions.
I believe people are better off if they can organize themselves around a goal. If someone has not decided where they want to go, they can become a victim of the caprices of life's ever-shifting winds.
People who don't have a mental illness are generally programmed to seek success in their work, fulfilling relationships, marriage, a house, a dog and children. Someone with mental illness may find that all of these things are beyond reach. If so, what is to be our life's purpose? This lack of direction can create a great deal of depression, substance abuse, or medication noncompliance. People with mental illness apparently are taught not to try. When we do make an effort to get somewhere, there are multiple avenues of sabotage and discouragement.
The negative shift that some persons with mental illness experience may happen to them in their thirties or forties. When in one's twenties, a person with mental illness hasn't reached the point of realizing that they might have limited prospects. Up to a point, a person still seems optimistic, and still has juice to continue facing challenges. At some point, a person may become tired of trying, and they may just give up.
A person can seem nice enough, and then at some point, they have lost that niceness and have become unable to live among people. It doesn't seem to me like people "snap." Instead, they experience a fairly abrupt downturn, like an aircraft losing altitude. In relatively rare instances, this is accompanied by the person becoming violent. While in other instances, the person just fails to continue doing the basic life maintenance and self care that people must do.
When someone has a loss of functioning like this in middle age, it may lead to long term institutionalization.
On the other hand, there are people with mental illness who improve with age. They are past the worst of their illness and have found a reason to continue. They might be living for that morning coffee or for the sight of a blue jay hanging out in a tree. They might be doing little things that interest them like long walks along the trails, sometimes with a friend. They might have found a lifetime partner, and then again might not.
The rule concerning people with mental illness is that you can't generalize. There is a whole range of possible outcomes, and these outcomes are the responsibility of individuals to forge for themselves. Someone who is external to a person with mental illness can not change the insides of that person. They can encourage and they can assist. And that is all they can do. There is no replacement for someone's willingness to keep trying.