Eos, Goddess of Dawn fell in love with a youth named Tithonius, and the two spent many happy years together. But while Eos being a goddess retained her youth, Tithonius began to age. He asked his beloved to grant him immortality. She couldn’t do it on her own so she pleaded his case with Zeus, the supreme deity.
Zeus reluctantly granted the boon.
The story unfortunately does not have a happy ending. Tithonius had forgotten to ask for eternal youth. So though he could not die he could age. As his age advanced he became wrinkled and hunchbacked and revoltingly ugly. He pleaded with Eos to help him.
She could not take back the gift of immortality nor could she give him back his youth. But she could change his form. She turned him into a grasshopper.
Source: English for Students