Age

• The vast majority of people who qualify for and use Medicare are those aged 65 or older. If you are already receiving Social Security or railroad board retirement benefits, you will be automatically enrolled in Medicare Parts A and B around your 65th birthday. If not, you will need to contact the Social Security Administration three months before your 65th birthday to enroll.
  
  

Citizenship

• Medicare beneficiaries need to be citizens of the United States before they can enroll. You may also enroll in Medicare if you are permanent resident.
  
  

Premiums

• Part A Medicare is available premium free to certain individuals. If you are not eligible for free Part A, you can still enroll if you 65 or older and pay a monthly fee. The costs depend on how long you have paid Medicare taxes in the past. As of 2011, people who have paid 30 to 39 quarters of Medicare taxes have a monthly premium of $248. Those who have fewer than 30 quarters of Medicare taxes pay $450 per month. Everyone who enrolls in Part B Medicare pays a monthly premium. In 2011, the Part B premium for new enrollees was $115.40.
  
  

Medicare Tax Payments

• Once you turn 65, you qualify for premium-free Part A Medicare if you have paid Medicare taxes for a minimum of 40 quarters, or 10 years. If you have not, you can still get Medicare if you pay monthly premium payments, which are based on your income.
  
  

Marriage

• If you are 65 or older and married to someone who has made sufficient payments into Medicare, you qualify for Medicare as well, even if you have not worked at a job where you paid Medicare taxes for 10 years.
  
  

Disability

• Even if you are under 65 years of age, you may qualify for Medicare if you have a disability of certain health conditions. Citizens with end-stage renal disease, kidney failure requiring dialysis, can enroll in premium-free Part A, as can those who receive disability benefits from the Social Security Administration or a disability pension form the railroad retirement board. You can also enroll in Medicare under 65 if you are receiving Social Security benefits because you have amyotrophic lateral sclerosis, better known as Lou Gehrig's disease.