Familiarity with good men is very advantageous to all, especially to the young, as is shown by the example of Joshua and Moses and others. Further, those who are unlike in age are often alike in virtues, as Peter and John prove.
97. It is a very good thing to unite oneself to a good man. It is also very useful for the young to follow the guidance of great and wise men. For he who lives in company with wise men is wise himself; but he who clings to the foolish is looked on as a fool too. This friendship with the wise is a great help in teaching us, and also as giving a sure proof of our uprightness. Young men show very soon that they imitate those to whom they attach themselves. And this idea gains ground from the fact that in all their daily life they grow to be like those with whom they have enjoyed intercourse to the full.
98. Joshua the son of Nun became so great, because his union with Moses was the means not only of instructing him in a knowledge of the law, but also of sanctifying him to receive grace. When in His tabernacle the majesty of the Lord was seen to shine forth in its divine Presence, Joshua alone was in the tabernacle. When Moses spoke with God, Joshua too was covered by the sacred cloud. The priests and people stood below, and Joshua and Moses went up the mount to receive the law. All the people were within the camp; Joshua was without the camp in the tabernacle of witness. When the pillar of a cloud came down, and God spoke with Moses, he stood as a trusty servant beside him; and he, a young man, did not go out of the tabernacle, though the old men who stood afar off trembled at these divine wonders.
99. Everywhere, therefore, he alone kept close to holy Moses amid all these wondrous works and dread secrets. Wherefore it happens that he who had been his companion in this intercourse with God succeeded to his power. Worthy surely was he to stand forth as a man who might stay the course of the river, and who might say: “Sun, stand still,” and delay the night and lengthen the day, as though to witness his victory. Why?— a blessing denied to Moses— he alone was chosen to lead the people into the promised land. A man he was, great in the wonders he wrought by faith, great in his triumphs. The works of Moses were of a higher type, his brought greater success. Either of these then aided by divine grace rose above all human standing. The one ruled the sea, the other heaven.
100. Beautiful, therefore, is the union between old and young. The one to give witness, the other to give comfort; the one to give guidance, the other to give pleasure. I pass by Lot, who when young clung to Abraham, as he was setting out. For some perhaps might say this arose rather owing to their relationship than from any voluntary action on his part. And what are we to say of Elijah and Elisha? Though Scripture has not in so many words stated that Elisha was a young man, yet we gather from it that he was the younger. In the Acts of the Apostles, Barnabas took Mark with him, and Paul took Silas and Timothy and Titus.
101. We see also that duties were divided among them according to their superiority in anything. The elders took the lead in giving counsel, the younger in showing activity. Often, too, those who were alike in virtue but unlike in years were greatly rejoiced at their union, as Peter and John were. We read in the Gospel that John was a young man, even in his own words, though he was behind none of the elders in merits and wisdom. For in him there was a venerable ripeness of character and the prudence of the hoarhead. An unspotted life is the due of a good old age.
Source: On the Duties of the Clergy (New Advent)