The Feds have mandated seatbelts in all cars since 1963, and I've been wearing them in every car that had em since I was 12 years old.
In 1962 my uncle was thrown from his Cadilliac thru the windshield and the car landed on top of him.
That's all it took to make my Dad believe in seatbelts, and when he got his new 1963 Dodge "Demo" he would always where them.