In Romeo and Juliet, the youth are more directly responsible for the violence in the play. In fact, the play opens with a conversation between two young Montagues. Not only that, Sampson, the Montague, starts a fight by biting his thumb at a Capulet, which was a type of insult. Since the play not only opens with the youth, it opens with the youth starting a fight, it can be said that Shakespeare is pointing out that the youth are the ones primarily and directly responsible for the violence. However, that being said, Shakespeare also points out that it is the adults who are morally responsible for the start of the whole feud.
Even though Lords Capulet and Montague arrive in the middle of the scene to join in the fighting--the boys having already started the fight--Shakespeare points out that it is primarily the lords that the prince holds responsible. When the prince finally does arrive to break up the city riot the two families have started, he blames Lords Capulet and Montague, addressing them specifically, saying,
"Three civil brawls, bred of an airy word, by thee, old Capulet, and Montague, have thrice disturb'd the quiet of our streets."
At the end of the speech where the Prince threatens death if the families fight again, it is again Lords Capulet and Montague that he singles out for judgement, not the youths, saying,
"You Capulet; shall go along with me: and, Montague, come you this afternoon, to know our further pleasure in this case, to old Free-town, our common judgement-place."
In other words, the Prince is ordering Lords Capulet and Montague to privately be judged, perhaps fined, by him for their behavior. Hence, even though the youths are the ones beginning the violence, the older members of the cast are morally responsible for the existence of the feud.