Do priests always know their calling from an early age?

No. We read about the origins of a priestly vocation. Ottavio was dying. All medical help had failed. As he awaited death, he thought about St Philip Neri. Ottavio lived in Naples and knew the Oratorians there. But then it came into Ottavio's mind to add to his prayer the promise of becoming an Oratorian, if he recovered. Ottavio did get better. And he kept his word.