I don't think you can ever start too young. One of the first PD books that I ever came across was
What Color is Your Parachute. I know it's not really considered a 'Personal Development' book, but more of a 'job hunting' book, but it does have considerably more to offer. A good part of the book centers on finding your purpose in life. There is also a copy out there for teenagers, that your brother might be interested in:
What Color Is Your Parachute (for Teens).