The Fallout by S.A. Bodeen
Theme 1: You are the creator of your own destiny.
His sister Lexie, who was adopted, want's to find her real parents. After a long search she finds out her mother was in jail when she was born and was later killed. She fears of becoming like her mother and ending up in jail. Both characters however end up proving that you are the creator of your own destiny. Eli and Lexie help protect their family from their father, who they thought was dead.
Theme 2: Even if something may seem like the best, it may really be the worst.
The bad side to being able to make yourself younger is that if everyone never aged, the world would over populate. This helps prove that even if something may seem like the best idea, it may actualy leed to many more problems.