40 ILCS 5/12-131

An employee, upon withdrawal upon or after age 60 with 4 or more years of service, shall be entitled to a retirement annuity. Such annuity for an employee who entered service prior to July 1, 1971 shall be that provided, as of his age on the date of retirement, from the total of the accumulations described below: (a) the accumulation from employee contributions for service annuity on the date of withdrawal, improved by regular interest to the date when he enters upon retirement annuity; (b) the accumulation from the contributions by the employer for service annuity, on the date of the employee's withdrawal, improved by regular interest to the date when he enters upon retirement annuity.