Which group is NOT covered by Medicare?

Medicare is a federal health insurance program primarily designed for specific groups of individuals, and it does not typically cover all children under the age of 18. Instead, Medicare focuses on seniors aged 65 and older, individuals with disabilities (including those under 65), and those with certain conditions like end-stage renal disease (ESRD).

Children and adolescents generally qualify for different types of health coverage, such as Medicaid or the Children's Health Insurance Program (CHIP), which are specifically designed to meet the healthcare needs of younger populations. This distinction highlights why the group of all children under 18 is not included in Medicare coverage, making it the correct answer to the question.