Is it possible to find out how old is a planet, especially a rogue planet? I know that people measured the radioactive decays to determine Earth's age with some amazing accuracy, but what about interstellar planets be, they gas or terrestrial?
At the moment there is basically only one way. That is to associate the planetary-sized object with a cluster of stars or moving group of stars of known age.
That's basically it. If the planetary-sized object really can't be associated with another object, then only limits can be placed on its age by comparing it's luminosity to theoretical planet cooling models. But to use these models you need to know the mass!
So at the moment, the only "free-floating" planetary mass objects known (they could all be low-mass brown dwarfs actually) are those for which an age is estimated by association and therefore a (planetary) mass has been deduced from cooling models.