Beneath his point that time is a thief who robs young ladies of their naivete and youthful joie de vivre, the older fat man pronounces much more about himself than he does the concept of the brevity of youthful delight and innocence. His words to Leila are, indeed, telling of his cynicism and loss of youthful idealism that is like "little satin shoes that chased each other like birds."
It is because he himself is a disappointed romantic who has lost his own youth and its accompanying delights that he seeks to destroy Leila's innocent and youthful delight in her first ball. For, Leila becomes the target of his cynicism in order that he can steal from her the enjoyment of the moment. This is why he is yet on the dance floor at his age. Indeed, he speaks of himself and his personal losses, rather than the ingenue he holds when he presses himself to her as though to draw youth into his own being:
"And your heart will ache, ache"...because no one wants to kiss you now."
It is Leila's intuitive comprehension of the fat man's motives--"Why had he spoiled it all?"--that she rejects his cynical statements that youth and beauty are but temporal. This is why she rebuffs his condescending remark, "...you must'nt take me seriously, little lady" by saying, "As if I should!" Rather than becoming disillusioned and cynical herself, Leila chooses to enjoy the moment and delight in it for what it simply is. She "didn't even recognize him again."