40 ILCS 5/9-133.1
A retired employee retired at age 55 or over and who (a) is receiving annuity based on a service credit of 20 or more years, and (b) does not qualify for the automatic increases in annuity provided for in Sec. 9-133 of this Article, and (c) elects to make a contribution to the Fund at a time and manner prescribed by the Retirement Board, of a sum equal to 1% of the final average monthly salary forming the basis of the calculation of their annuity multiplied by years of credited service, or 1% of their final monthly salary multiplied by years of credited service in any case where the final average salary is not used in the calculation, shall have his original fixed and payable monthly amount of annuity increased in January of the year following the year in which he attains the age of 65 years, if such age of 65 years is attained in the year 1969 or later, by an amount equal to 1 1/2%, and by an equal additional 1 1/2% in January of each year thereafter. Beginning with January of the year 1972, such increases shall be at the rate of 2% in lieu of the aforesaid specified 1 1/2%. Beginning with January of the year 1982, such increases shall be at the rate of 3% in lieu of the aforesaid specified 2%. Beginning January 1, 1998, these increases shall be at the rate of 3% of the current amount of the annuity, including any previous increases received under this Article, without regard to whether the annuitant is in service on or after the effective date of this amendatory Act of 1997.
In those cases in which the retired employee receiving annuity has attained the age of 66 or more years in the year 1969, he shall have such annuity increased in January of the year 1970 by an amount equal to 1 1/2% multiplied by the number equal to the number of months of January elapsing from and including January of the year immediately following the year he attained the age of 65 years if retired at or prior to age 65, or from and including January of the year immediately following the year of retirement if retired at an age greater than 65 years, to and including January of the year 1970, and by an equal additional 1 1/2% in January of each year thereafter. Beginning with January of the year 1972, such increases shall be at the rate of 2% in lieu of the aforesaid specified 1 1/2%. Beginning with January of the year 1982, such increases shall be at the rate of 3% in lieu of the aforesaid specified 2%. Beginning January 1, 1998, these increases shall be at the rate of 3% of the current amount of the annuity, including any previous increases received under this Article, without regard to whether the annuitant is in service on or after the effective date of this amendatory Act of 1997.
To defray the annual cost of such increases, the annual interest income of the Fund, accruing from investments held by the Fund, exclusive of gains or losses on sales or exchanges of assets during the year, over and above 4% a year, shall be used to the extent necessary and available to finance the cost of such increases for the following year.
40 ILCS 5/9-133.1