You can enroll in a Medicare Supplement plan anytime, but the best time is during your initial enrollment period because you are guaranteed approval in your initial enrollment period. Most Medicare Supplement plans require underwriting if you are not in a guaranteed issue election period.
You're eligible for Original Medicare if you or your spouse paid into Social Security for at least 10 years through employment, you are a citizen or permanent resident of the U.S., and you are age 65 or older.
The full Social Security retirement age varies based on the year you were born (age 66 for those born 1943 to 1954), but you're eligible for Medicare at age 65. You do not have to be retired to enroll. If you'll have employer-provided health coverage at age 65, contact your benefits administrator (typically part of your human resources department) to find out how Medicare will work with your current coverage.