In Chapter 9 of The Chrysalids, David is shown to be nearly a grown up because the time is established as six years after his aunt's death. Also, David's reactions are more mature and experienced. When he is working in the fields, another indication of his maturity, and he hears Petra's (though unidentified at the time) cries of distress, he drops everything and obeys an impulse he has learned to recognize, know and trust, although stronger than he has ever experienced it to be.
After he rescues Petra from drowning, he answers the interrogation of the townspeople with a mature and practiced deception and evasiveness that is guilt-free from years of practice in self-preservation. Another indication is that he logically and rightly puzzles through the problem of how to deal with communicating about telepathy with Petra. Finally, David has nightmares about a purification ceremony, but the victim isn't himself...it is Petra whose preservation he now lovingly places above his self-preservation.