The answer
Can I avoid the Part B penalty?
Yes, you can avoid the Part B (medical) penalty by enrolling on time or by having qualifying coverage that lets you delay.
The simplest way is to sign up during your Initial Enrollment Period, the seven-month window around your 65th birthday. If you keep working past 65 and have active employer coverage through a job with 20 or more employees, you can delay Part B without penalty and enroll later using a Special Enrollment Period, which gives you 8 months after that employment or coverage ends, whichever comes first. Watch out: retiree coverage, COBRA, and marketplace plans do not count as qualifying coverage for this purpose, so delaying with those can trigger the penalty.