In The Chrysalids, David goes from an innocent little boy to a young man with adult responsibilities.
In the beginning of the story, David reflects on a moment that shaped his childhood. It was when he met the little six-toed Sophie, and realized that people with deviations were not monsters. This is important because David himself is a deviant. He can see thought-pictures shared by certain others, including his cousin Rosalind.
I was a normal little boy, growing up in a normal way, taking the ways of the world about me for granted. … It is hind-sight that enables me to fix that as the day when my first small doubts started to germinate. (ch 1)
The other event that dramatically changes David’s life is when he realizes that Petra is also telepathic. From that point on he has to worry about someone other than himself. He has to protect Petra, and take on a rather adult responsibility.
The next day I tried to send thought-shapes to Petra. It seemed to me important for her to know as soon as possible that she must not give herself away. I tried hard, but I could make no contact with her. (ch 9)
Petra is too young to protect herself, and too young to understand what is happening to her. David must look out for her to keep anyone else from finding out about her, but since she is so strong she is also a threat to all of the telepaths.
It is Anne’s choice that causes all of the telepaths to have to flee though. When she marries a normal man, everyone is in danger. She loves him, so she chooses to tell him. He cannot accept it, and Uncle Axel has to kill him to protect all of the telepaths. It doesn’t work. They are found out. David and Rosalind flee with Petra.
On the run, David experiences many adult situations. His love for Rosalind is developing. He has to have a parent-like relationship with young Petra. He tries to protect Rosalind from his uncle’s less-than-pure intentions.
David has become hardened and matured. He is no longer the little boy he was.