Eligibility for award, commencing with the period after 31 December 1960, was established by DOD Directive 1348.7, dated 1 April 1966, and terminated effective 15 August 1974, per letter from Manpower and Reserve Affairs, subject: Termination of Eligibility for the National Defense Service Medal, dated 30 June 1974.
If you are single and under age 65, you can earn up to $9,499 in a year and not file a tax return. Should you be 65 or older, you could earn up to $10,949 and be exempt from filing a federal tax return. However, you may qualify for an Earned Income Tax Credit, which is refundable in cash to you.
1 In the year you reach full retirement age, we deduct $1 in benefits for every $3 you earn above a different limit, but we only count earnings before the month you reach your full retirement age.  If you will reach full retirement age in 2016, the limit on your earnings for the months before full retirement age is $41,880. Starting with the month you reach full retirement age, you can get your benefits with no limit on your earnings.