Retirees tend to use mental health services less than the general population. On the bright side, people get happier as they age and may not need a therapist. However, treatment is also expensive, and retirees may be unaware that Medicare covers psychiatrists, psychologists, and in-patient and out-patient treatment. Retirees with Medicare Advantage insurance policies face another obstacle: ghost networks of mental health professionals that the insurer claims to cover. A new report by the Senate Finance Committee, based on a sampling of 12 Advantage plans in six states, determined that only about 20 percent of the people with these policies who seek services would be able to make an appointment because a majority of the plans’ lists of in-network therapists…