A Montana driver license that is obtained by a driver on his or her 18th birthday will expire on his or her _____ birthday.

How long your driver license will be valid depends on your age when it is issued to you. If you are under 21 years of age when it is issued to you, your driver license will expire on your 21st birthday. [Fees/Receipts, Chapter 1 – The Driver License, Montana Driver Manual]