The Fourteenth Goldfish by Jennifer L. Holm

When Ellie's mom brings her grandfather home from the police station, Ellie does not really question his appearance, but she should have!  The brilliant scientist is no longer an old man, somehow he has discovered the secret to eternal youth and is now a teenage boy.  Because of his brush with the law, the teenage "long lost cousin" is now required to attend school with Ellie.  While her grandfather tries to enlist Ellie's help in breaking into his lab, to recover his ground breaking research, Ellie questions everything she thought she knew about science and life.  Will Ellie be able to deal with her embarrassing grandfather at middle school?  Will her grandfather finally get the fame and recognition he has worked so hard to accomplish?  What would life be like if everyone could be young again?