It's all about the MRA.
http://www.opm.gov/employ/vera/vera01.asp
A FERS Annuity Supplement is payable to an employee who has completed at least one calendar year of FERS service when he/she reaches Minimum Retirement Age (MRA). MRA is age 55 to 57, depending on date of birth. The annuity supplement is payable until eligibility for Social Security begins at age 62, subject to an earnings limitation.
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