[AvengerRam]

Let's get rid of the easy issue first - it should be mandatory for minors, without question.

Now, for adults. Should the government prevent adult drivers from taking the needless risk of driving without a seatbelt?

My answer: yes.

While GC is right that there are plenty of things that people do (i.e. smoking) that are contrary to self-interests, there is one big difference when it comes to driving - driving is a privilege which is granted by license from the government (in exchange for the government's agreement to build, maintain and police safe roadways). The government can set reasonable criteria to obtain and maintain licenses. Mandatory seatbelt usage is a very reasonable requirement.