An increasing number of Americans are retiring in Mexico to places like Puerto Vallarta, and for a variety of reasons, are losing health insurance between the ages of 55 and 65 before they qualify for Medicare. In fact, people between the ages of 55 and 64 are losing coverage faster than any other age group. An estimated 3 million uninsured Americans in the 55 to 64 age group are early retirees whose former employers have reneged on promises to guarantee coverage until age 65--an unanticipated consequence of massive corporate downsizing in recent years. Others are spouses of . . .