How are benefits for FERS “special group” employees calculated?
Law enforcement officers, firefighters and air traffic controllers may voluntarily retire at age 50 and 20 years or more of covered service, or any age and 25 or more years of such service. The former two groups generally are subject to mandatory retirement at age 57, the latter at 56.

The annuity is 1.7 percent of high-3 pay multiplied by years of service up to 20 plus 1 percent times all remaining service. There is no annuity reduction for retiring under age 55.