The Picture of Dorian Gray mentions a panegyric on youth. What does that mean?

A panegyric is a formal expression of praise, sometimes for the dead. In the novel, Dorian Gray views his newly finished portrait and is astounded by his own physical beauty. But like a knife in his heart, he recalls his mentor's speech — a panegyric — about youthful beauty and how it quickly disappears:

When he saw [his picture] . . . the sense of his own beauty came on him like a revelation. He had listened to [some earlier compliments], laughed at them, forgotten them. Then had come Lord Henry Wotton with his strange panegyric on youth, his terrible warning of its brevity. ... Yes, there would be a day when his face would be wrinkled and wizen, his eyes dim and colourless . . . He would become dreadful, hideous, and uncouth.