Mont. Code Ann. § 33-20-509

For the purpose of determining the benefits calculated under 33-20-507 and 33-20-508, in the case of annuity contracts under which an election may be made to have annuity payments commence at optional maturity dates, the maturity date is the latest date for which election is permitted by the contract but may not be later than the anniversary of the contract next following the annuitant's 70th birthday or the 10th anniversary of the contract, whichever is later.