The Fourteenth Goldfish by Jennifer L. Holm

Middle school is usually pretty terrible, and for 11-year-old Ellie, it certainly starts out that way. To begin with, her best friend has discovered volleyball, and through volleyball, new friends to eat lunch with so Ellie eats alone. Furthermore, Ellie is uninterested in sports, or theater, which is the profession of her divorced parents. Hope comes in the form of her brilliant scientist grandfather Melvin, who has, with the help of regenerative jellyfish genes, cured aging! Melvin is a young boy again, and becomes Ellie's closest friend, even though he still dresses like a crotchety old man. Together they discuss the ethical ramifications of science, and how far one ought to go with it.