The old man serves as a reminder that life is not perfect. Even in Leila's dreamy environment, there is a reality check when the old man claims that Leila will too grow old and find herself sitting with the old women on stage who are not dancing.
The old man is a realistic picture of life. He casts a shadow on Leila's perfect evening. He ruins her evening momentarily. He brings Leila back down to earth. She was floating on ecstasy, but the old man reminds her that life is not always glamorous. Eventually, the young at heart will grow old and have to face the reality of old age. Although this is all true, Leila did not have to be reminded of such a dreadful description of old age on her magical evening:
Leila laughs at the fat man's words, though they bother her inwardly because she realizes that they are essentially true. One day she will grow old; then no one will dance with her, and she will become one of the chaperons.
In every fairy tale, there is also a dreadful person who tries to steal someone's happiness. The old man serves as Leila's evil doomsday speaker. He serves as the wicked stepmother or evil queen. Leila's perfect evening is ruined until she meets the young man with curly hair. Dancing with him brings back the magic she is enjoying, even if it is only temporary.