When I was sixteen, I was really worried about changing as I grew older and losing the person I was right then. Two things that made me feel better were: the movie, Dark City, and the book, "A Miracle of Rare Design" by Mike Resnick. They're both stories about maintaining the integral core of yourself despite undergoing vast changes -- in Dark City the main character repeatedly has his memories changed or erased, but he manages to stay himself through it all. In "A Miracle of Rare Design," the main character is physically changed into all different kinds of aliens through the course of the plot which affects the way he thinks... It's a really neat book. And I found it particularly uplifting and hopeful when I used to worry about changing as I aged.