Mont. Code Ann. § 19-7-502

A member with at least 5 but less than 20 years of membership service may retire with a retirement benefit commencing no sooner than the first day of the month following the member's 50th birthday. The retirement benefit must be calculated to be the actuarial equivalent of the member's service retirement benefit as otherwise accrued, based upon payment commencing when the member would have completed 20 years of membership service or reached age 60, whichever event would have occurred first.