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.
According to efile.com, the amount of money a person can make without filing taxes depends on filing status and age. For tax year 2013, a single person under 65 years old had to make in excess of $10,000 before filing taxes, while someone over 65 years old needed income over $11,500 to file taxes. Dependents have a lower tax filing threshold of $6,100 if they're under 65 years old, single and not blind.
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.