Enrolling in Part D Prescription coverage is voluntary unless receiving benefits from Medicaid.
If you already have sufficient (creditable) prescription coverage through a group plan or VA benefits then it is not necessary to join a Part D plan.
If you don’t sign up for Prescription Drug coverage when first eligible or have some other creditable coverage you will be charged a late enrollment penalty. In addition you can be penalized anytime you go a period of more than 62 days without creditable coverage. The penalty is calculated by multiplying 1% of the national base premium times the number of months you should have been enrolled but did not.