I'm not a fan of "big brother" laws like seatbelt laws. However, minors are technically wards of the state as far as the law is concerned. Therefore, IMO, a state should require that people under the age of 18 wear seatbelts. Once you're an adult, it should be a matter of personal preference. However, health insurance companies should be able to jack up your rates or deny coverage if you get injured in a motor vehicle accident while not wearing a seatbelt.
I once got into an accident in an older vehicle without seatbelts. I was very lucky to have been thrown from a vehicle at 65 mph and literally walk away with only minor injuries.