40 ILCS 5/14-126
Any member having 15 or more years of creditable service, and having attained at least age 55, or having 20 or more years of creditable service and having attained at least age 50, who, after receiving nonoccupational disability benefit for the maximum period of time specified herein is still disabled for service, shall be entitled to receive a retirement annuity beginning the first of the month following application, without regard to whether the member has attained age 60.
If a member having 15 but less than 20 years of creditable service is under age 55 when nonoccupational disability benefits terminate, and the member has been continuously disabled for service, the member is entitled upon application to the retirement annuity upon the first of the month after attainment of age 55.
If a member having 20 or more years of creditable service is under age 50 when nonoccupational disability benefits terminate, and the member has been continuously disabled for service, the member is entitled upon application to the retirement annuity beginning upon the first of the month after attainment of age 50.
As an option to the computation of a retirement annuity in the manner provided in this Article, if a person who retires on a retirement annuity prior to age 60 under the provisions of this Section re-enters State employment, that person may refund to the system the amount theretofore received as a retirement annuity and upon subsequently retiring from State service shall be entitled to a retirement annuity computed as though that member had not previously received such annuity.
40 ILCS 5/14-126