Each year, an obscure federal rule forces thousands of seniors to pay penalties for the rest of their lives simply because they made an honest mistake while signing up for Medicare. That number will rise unless Congress modernizes the program’s enrollment process, which is tethered to outdated ideas about seniors and retirement. Seniors are twice as likely to be working today as in 1985. So many more of them have to decide whether to sign up for Medicare when they become eligible at age 65 or keep their job-based health plan. Making the wrong decision can be costly.