Tuck Everlasting by Natalie Babbitt

Doomed to - or blessed with - eternal life after drinking from a magic spring, the Tuck family wanders about trying to live as inconspicuously and comfortably as they can. When ten-year-old Winnie Foster stumbles on their secret, the Tucks take her home and explain why living forever at one age is less a blessing that it might seem. Complications arise when Winnie is followed by a starnger who wants to market the spring water for a fortune