40 ILCS 5/6-124

When a future entrant attains age 63 in service, except for a fireman who is not subject to the compulsory retirement age, his age and service annuity shall be fixed as of age 63. The annuity shall be that provided from the entire sum accumulated to his credit for age and service annuity on the date he attains age 63. When a future entrant who is not subject to the compulsory retirement age withdraws from service and is at least age 63, his or her age and service annuity shall be fixed as of the age he or she withdraws from service. The annuity shall be that provided from the entire sum accumulated to his or her credit for age and service annuity on the date he or she withdraws from service.