Mercer L.L.C. said it is launching a new health insurance exchange in which retired employees eligible for Medicare can choose from an array of different health care plans.While the prevalence of employers offering coverage to Medicare-eligible retirees continues to decline, about one-third still do, according to Mercer.Through an exchange, employers no longer directly offer coverage, relieving them of a significant administrative burden while giving retirees more benefit choices, Mercer notes. In addition, the exchange, which Mercer is offering along with Connextions, Inc., a technology solutions company, will provide assistance to retirees during enrollment periods and throughout the year.