Jerome

"Rejoice, O young man, in your youth; and let your heart cheer you in the days of your youth, and walk in the ways of your heart, and in the sight of your eyes: but know, that for all these things God will bring you into judgement. Therefore remove sorrow from your heart, and put away evil from your flesh: for childhood and youth are vanity. "In this chapter there were many explanations of all things and almost as many opinions as men themselves. …

Commentary on Ecclesiastes ↗

Gregory the Dialogist

As he doth, in the first place, express the delight of carnal things, and pronounceth it to be good to cast away all care, and to eat and drink: so afterward, with reason and judgment doth he reprove that, when he saith that it is better to go unto the house of mourning, than to the house of feasting: and though he saith that a young man ought to rejoice in his youth, yet doth he utter that as proceeding from the resolution of a carnal mind; seeing afterward, by definitive sentence, he reproveth both youth and pleasure, as vain …

Dialogues, Book 4, Chapter 4 ↗

Bonaventure

Rejoice therefore, O young man, etc. Here secondly he reproves present joy, asserting that man must be judged for it; whence speaking ironically he says: Rejoice therefore, O young man, in your youth, because that age is suited to joy; whence Wisdom 2: "Come, let us enjoy the creature as swiftly as in youth"; as if to say: do not rejoice in youth, lest you grieve in old age; whence Ecclesiasticus 5: "Do not follow the desire of your heart in your strength." And he explains this joy, so that it may be complete both in mind and in deed …

Commentary on Ecclesiastes, Chapter 11 ↗