The title relates to the everlasting life the Tuck family has because they drank from the Fountain of Youth.
This novel is about a family who found a spring that turned out to be the Fountain of Youth, or a fountain of youth. When you drink from this spring, you become immortal, meaning you never die and have everlasting life. Although this sounds nice on the surface, it is actually more of a curse than a blessing for the Tuck family.
When they drank from the spring, they had no idea what they were doing. They spend the rest of their immortal lives making sure no one else drinks, so they do not make the same mistakes they did.
When Winnie sees Jesse Tuck drinking from the spring and insists on drinking too, he has no choice but to kidnap her and take her to his family so that they can explain. He cannot risk her actually drinking the water without making the choice and knowing what she is doing. When the family tells the whole story, Mae explains that the family realized they had to prevent anyone from knowing what was in the water.
"And we figured it'd be very bad if everyone knowed about that spring," said Mae. "We begun to see what it would mean." She peered at Winnie. "Do you understand, child? That water—it stops you right where you are. …" (Ch. 7)
The family is stuck off the wheel of life. While time goes on, they sit still. They do not change, or grow, or even grow up. So if Winnie drank the water, she would be a child forever in addition to living forever. This is a little bit different from everlasting life. It would be an everlasting childhood.
In some ways, Jesse and his brother Miles experience the benefits of everlasting youth. However, they can never have real families, because their children will grow up and their wives will grow old without them. One of the biggest disadvantages of the spring is loneliness for the Tucks.
"We've never had anyone but us to talk about it to. Winnie—isn't it peculiar? And kind of wonderful? Just think of all the things we've seen in the world! All the things we're going to see!" (Ch. 8)
The Tucks can never get too close to anyone, because people will wonder why they never age. The boys cannot have families, and while Ma and Pa Tuck are married, the boys go off and leave them often, finding work because they are bored and want something like a normal life.
This book explores the consequences of too much of a good thing. Would you really want to live forever? Maybe not. As with most things like this, someone of course wants to exploit the stream and the family. Not everyone who thinks about the question comes up with the same answer.