In chapter fourteen, Jesse Tuck suggests to Winnie that she should drink water from the special spring water. But he also wants Winnie to wait a little bit before drinking from the spring. Jesse wants Winnie to wait until she turns seventeen years old. That way both Jesse and Winnie can be the same age for the rest of eternity.
"But the thing is, you knowing about the water already, and living right next to it so's you could go there any time, well, listen, how'd it be if you was to wait till you're seventeen, same age as me—heck, that's only six years off—and then you could go and drink some, and then you could go away with me!"
Jesse's next suggestion to Winnie is that he and Winnie could get married in the future. Jesse figures that if both of them are immortal teenagers, it only makes sense that they should get married.
"We could get married, even. That'd be pretty good, wouldn't it! We could have a grand old time, go all around the world, see everything."
As a reader, I never found Jesse and Winnie's budding romance as believable. Regardless though, Jesse suggests that Winnie waits to drink the water until she is seventeen, because then they can be married together forever.