Sec. 12. SERVICE RETIREMENT. (a) A member of a retirement system is eligible for retirement for service if the member is at least 55 years old and has performed at least 20 years of service that is credited in the retirement system.

(b) Except as provided by Subsection (c) of this section, monthly benefits payable for service retirement are $100 to a retiree whose service was not exclusively as a volunteer and $25 to a retiree whose service was exclusively as a volunteer.

(c) A board of trustees established under this Act may, in accordance with Section 7 of this Act:

(1) decrease the age or service requirements for service retirement from a particular retirement system; or

(2) determine formulas for computing benefits, classes of permissible beneficiaries, and other requirements for payment of service retirement benefits, as long as the minimum benefits payable to a retiree are not less than the amounts provided by Subsection (b) of this section.