While most people newly eligible for Medicare are automatically enrolled in Part B—because they are collecting Social Security retirement benefits at or before age 65—a growing number are working later in life and deferring their Social Security benefits. Unlike those who are auto-enrolled, these individuals must make an active Medicare enrollment choice, taking into consideration specific timelines and existing coverage. If this transition is mismanaged, individuals new to Medicare may face lifetime late enrollment penalties, higher health care costs, gaps in coverage, and disruptions in care continuity.