Ind. Code § 10-12-3-6

Sec. 6. If an employee beneficiary is retired for old age, the employee beneficiary is entitled to receive a lifelong monthly income as specified in section 7 of this chapter. However, to be entitled to the full amount of the basic pension amount, an employee beneficiary must have completed at least twenty (20) years of service to the department before retirement. Otherwise, the employee beneficiary is entitled to receive a proportionate pension based on the employee beneficiary's years of service to the department.