My daughter is 16 and her boyfriend is 19. What is the age in the state Montana before it does not fall under statutory rape.

In Montana the age a minor can legally consent to sexual activity is 16.

It is considered "non-consensual" and is a criminal sexual offense if the minor is under the age of 16 and the perpetrator is 3 or more years older.
45-5-503 Sexual intercourse without consent [1]