Which detail contributes to the irony in “The Colomber” by Dino Buzzati?

“It’s the fish that sailors fear above all others . . . .”
“When Stefano Roi was twelve years old . . . .”
“‘Despite my forty years, . . . I believe I still have good eyesight.”
". . . as soon as he was a man, he found a dignified and well-paying position at an emporium . . . ."