How was Boruto able to throw the Rasengan and use Gentle Fist? In the movie, it is stated that he threw the Rasengan and was able to make it vanish and reappear. Also while watching the movie, in one of his fights in the third match, he was in the form of the Hyuga's family Gentle Fist's way. At the point in the movie where he throws the Rasengan at the Hyuga's member, he had made it vanish to reappear and explode.

How is it possible at his age?
Boruto has the Hyuga blood within him therefore he has been trained in the art of the gentle fist by his mother and grand father. Being a prodigy he has already learned it.

Boruto's Rasengan is not like Naruto's Rasengan. Boruto accidentally added added a change of Nature transformation along with the change of shape transformation needed for a pure Rasengan. This makes his Rasengan more like Naruto's Rasenshuriken. But unlike Naruto's Rasenshuriken which adds the wind element to the Rasengan to transform it into a shuriken and attack the target, he subconsciously adds Lightning Release to it resulting in the technique disappearing from sight.