40 ILCS 5/6-123
Any present employee who withdraws on or after the effective date, having at least 20 years of service, and for whom the annuity otherwise provided in this Article is less than the amount stated in this section, has a right to annuity as follows:
If he is at least age 50 on withdrawal, his annuity, from and after such withdrawal, shall be 50% of his salary on the day one year prior to such date.
If he is less than age 50 on withdrawal, his annuity, after the date he becomes age 50, shall be 50% of his salary on the day one year prior to the date of his withdrawal.
Any such employee who remains in service after qualifying for annuity under this section or Section 10-9-53 of the Firemen's Annuity and Benefit Fund of the Illinois Municipal Code, shall have added to his annuity an additional 1% of salary for each complete year of service or fraction thereof accruing until July 21, 1959, and an additional 1% for a total of 2% of salary after July 21, 1959. "Salary" as referred to in this paragraph shall be determined by striking an average of the 5 consecutive highest years of salary within the last 10 years of service immediately preceding withdrawal.
40 ILCS 5/6-123