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If you are younger than full retirement age during all of 2013, the SSA will deduct $1 from your benefits for each $2 you earned above $15,120. These rules apply until you reach your full retirement age of 66.
There is a special rule that applies to earnings for one year, usually the first year of retirement. Under this rule, you can get a full Social Security check for any whole month you are retired, regardless of your yearly earnings.
Note that these deductions in benefits aren't lost. If some of your retirement benefits are withheld because of your earnings, your benefits will be increased starting at your full retirement age to take into account those months in which benefits were withheld.