What does it mean to grow up?

Growing up means having to make difficult choices, keep secrets, live with the consequences of one’s actions, realize that the only constant in life is change—and, where the ultimate change is concerned—accept death as a natural part of the life cycle rather than a fearsome inevitability. In Babbitt’s novel, we see what it means to grow up through Winnie’s character.
At the beginning of the story, Winnie, a child who feels oppressed by her family, runs away to the woods. Her decision to do so leads to a fateful meeting with the Tuck family. She learns about the spring of immortality in the woods, and she swears an oath of secrecy about the Tuck family’s unusual condition. As they are immortal, all have been forced to live a life of isolation and secrecy for over eighty years: since they can neither age nor die, they never remain in the same town or place for very long.
Unfortunately, information about the spring’s magical properties has been overheard by a sinister character, the man in the yellow suit, who desires to market the spring water. After bargaining Winnie’s safe return for ownership of the spring, the man returns to the Tuck residence, reveals his plan, and tries to make Winnie drink the water. Mae kills him but she is arrested by the constable shortly thereafter.
Although Winnie helps free Mae to preserve the family’s secret and ultimately, save the world from the ravages immortality would wreak upon its population, she has still broken the law. As such, her crime now forces the Tucks to leave the woods and live their lives on the run. Before they leave, Jesse gives Winnie some of the spring water. Naively believing that she can access the spring any time she wants—the forest and spring will always be there, after all—Winnie chooses not to drink it.
Over time, a lightning strike destroys the woods, and the spring is lost. When the Tucks finally return for a visit, Winnie, who has never partaken of the spring water, has succumbed, as must all who live within the natural order, to time and death.