10 years old.

We don't have sales tax in MT, so new cars are taxed pretty good on the first few years licenses. As their value decreases, so too the tax paid when you license them. By the time ordinary cars are 10 years old, the value and license is pretty cheap. If you want to do the 10 year deal at that time, multiply the regular fee by 3 and that's the last license you'll ever buy for the car.

This only works for cars and light trucks. Old 2 ton box-trucks have to be licensed every year