Medicare covers people who are 65 or older or people who’ve qualified due to a particular disability or end-stage renal disease. If you qualify for Medicare, there’s no need to apply for ACA health insurance since you’re already covered.
If your household includes younger family members who aren’t covered by Medicare, they’ll need to have their own health insurance either through an employer, Medicaid or even a policy they purchase on their own. If they don’t get coverage, they’ll have to pay a penalty (unless they’re exempt.)
Remember, under the Affordable Care Act there’s a “no wrong door” policy. This means that, no matter where you shop for insurance, you should get directed to the right resources for your family’s unique needs. When you visit a state marketplace, the federal marketplace, or a private marketplace (like GetInsured), you’ll be asked questions about your age, household size, and household income. If your answers suggest that you might qualify for Medicare, you’ll be directed to a resource where you can enroll.