What Mae, Jesse, and Miles tell Winnie is the strangest thing she's ever heard. They explain that 87 years ago, they all came west and found the forest. They'd planned to start a farm on the other side, but the forest went on for miles. Finally, they got to the ash tree in the center, and everyone, including the horse but except for the cat, drank from the stream. On the other side of the forest, they started their farm. Then, strange things started to happen. Jesse fell out of a tree, hunters shot the horse on accident, and a snake bit Angus--but they didn't die.
The origins of the Tucks' story functions to establish them as normal people, looking to start a life somewhere in the American Midwest. In other words, they begin this way in an attempt to show Winnie that they're not scary or different from anyone else; they want to farm and have families, and they have accidents and mishaps like all people do. It’s particularly notable that they never chose to change their lives in this way; the transformation happened essentially by chance.
Though this was worrying enough, even stranger was that none of them were aging. When Miles was about 40, he still looked 22. His wife decided he'd sold his soul to the devil and left with their children, and the family's friends started to suggest that the Tucks were involved with witchcraft. The family started to wander east again, came through Treegap and found the wood, and returned to the clearing in the wood. The tree looked exactly the same as it had twenty years ago, and they remembered that they all drank from the stream. The cat, the only one who didn't drink, had died ten years before, and they figured they weren't aging because of the water. To test this, Angus shot himself through the heart. He didn't die.
The fact that Angus is the one to test the stream's efficacy by shooting himself reinforces that Angus is the Tuck who's most uncomfortable with eternal life--he's uncomfortable enough with it to try to take his own life. Pay attention too to the trauma that Miles certainly experienced when his wife took his children. This reminds the reader that immortality isn't actually wonderful; it deprives Miles of the ability to watch his children grow and develop, and he lost the woman he loved because of it.
Jesse explains that they all went crazy for a while, excited that they'd live forever, but soon realized that it'd be awful if everyone in the world found out about the stream. Mae looks Winnie in the eye and says that if she drank the water now, she'd stop growing right where she is. She'd be 10 forever, and she'd never grow up. Jesse says that Angus thinks that the stream is left over from "some other plan" for the world, and it somehow got forgotten. He says he was telling the truth earlier; he really is 104, but he's also going to be 17 forever.
Because of Winnie's youth and immaturity, she doesn't yet grasp the gravity of what it would mean for her to stop developing right where she is. At this point in her development, the world is her oyster and it may be appealing for her to want to stop there. However, stopping there would also deprive her of experiences that would only be available to her as an adult, which is a point that Angus will go on to make later.