Dating the Sun is an indirect process. The age of the Sun can be estimated from the ages obtained from radioactive dating of the oldest meteorites. Therefore the age of the Sun should be close to the age of the meteorites, which can be found using the method of radioactive dating.
Additional evidence comes from the Earth.
The oldest Earth rocks are also about 4.6 billion years old. The oldest fossils, found in Australia, are about 3.5 bilion years old. The presence of fossils in rocks indicates that the Earth was a suitable place for life when the fossils formed. This implies that the Sun was luminous at that time. The model can be run forward and backward in time, and the astronomers can check the observable quantities in the model like luminosity, solar radius, composition, solar p-mode frequencies, and so on with our real Sun.