Working During Retirement

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Every dollar of earned income, up to any age, can result in increases in Social Security benefits. Social Security will automatically recalculate your primary insurance amount (PIA) every year in which you work. If one of your 35 highest indexed-earnings years is attained after you start benefits, then you will be credited with a higher benefit. However, you do not receive any help from annual wage indexing after the calculation made in your 60th year. As long as you keep paying FICA or SE (self employment) taxes, however, you can potentially keep increasing your benefit. There is no disincentive in the Social Security system to keep working after you start benefits.
Source: http://www.investopedia.com/articles/retirement/08/6-retirement-myths.asp