If you are a CSRS or CSRS Offset employee and retire before reaching age 55 due

to an involuntary separation, such as in a reduction-in-force situation, your

basic CSRS annuity  will be reduced by one-sixth of 1 percent for

each full month you were under 55  (2% per year).  

There is no age reduction if you retire under the disability provision or

under the special provisions for air traffic controllers, law enforcement

officers, and firefighters 

Example:
Age	52 years, 6 months
Years of Service	20 years
Months from 55th Birthday	26 months
26 x 1/6th of 1% (0.166667)	= 4.33% reduction in annuity

In this example, because the employee has served 20 years and is over 50, is eligible for early retirement, but will receive a reduction in their CSRS basic annuity by 4.33% if they retire at age 52 years and 6 months.

Source:  OPM  83-7