Mont. Code Ann. § 33-22-152

A health insurance issuer that issues or renews an individual or a group health insurance policy, certificate, or membership contract under which an individual's or employee's dependents are eligible for coverage may not terminate coverage on the basis of the age of an unmarried dependent, as defined in 33-22-140 (5)(b), prior to the dependent reaching 25 years of age. Except as otherwise provided by law, the continuation of the coverage of the dependent, as defined in 33-22-140 (5)(b), is at the option of the covered employee.