Yes they are but only in the front seats, not in the back.
Not if they did not come from the factory with seat belts. If they came form the factory with seat belts you are required to wear them.
Children up to certain weights are required to be in car seats that are held in place with the seat belts in most states. After they reach a certain weight, they are required to wear seat belts, so not only can children wear seat belts, after they no longer are required to be in car seats, they are required to wear a seat belt.
a seat belt for each seat
The 1965 came with lap belts for the front seat only. This was a new law for 1965 and only required front belts.
If the vehicle was built with seat belts you have to have them but if it didn't have the belts from the factory your not required to have them.
Everyone
Everyone
Whatever the vehicle came with when it left the factory is what is required.
In the United States, vehicles manufactured before 1968 are generally not required to have seat belts. The federal mandate for seat belts in passenger vehicles began with the National Traffic and Motor Vehicle Safety Act of 1966, which required all new cars to include seat belts starting in 1968. Therefore, vehicles from 1967 and earlier do not have to be equipped with seat belts. However, regulations can vary by state, so it's important to check local laws for specific requirements.
...save lives and are required by law.
Seat belts were invented in the 19th century by the English engineer George Cayley, but were not required to be installed in new cars in the US before 1959.
Sweden was the first country to make seat belts compulsory in 1970. The law required all drivers and passengers to wear seat belts in the front seats of vehicles.