In The Chrysalids, mention two examples that show that David is changing into an adult.
In The Chrysalids, David is used to ensuring that his secret is not revealed. He desperately wishes he was normal and has seen the impact of Sophie's secret and the mysterious death of his aunt, Harriet but is powerless to do anything.
Six years have passed since Harriet died and David is working in the fields, older and wiser. He recognizes his inherent power although he, at first, does not know what he is responding to, feeling a pressing need to find Petra his sister, whom he then saves from drowning. He knows that it is Petra's own power that must have reached him and his self-awareness shows his maturity and his ability to consider the consequences if he reveals too much to Petra or to anyone else. David carefully chooses his words and explains to the townsfolk that it was Petra's cries that he heard and that that, fortunately, alerted him and, in fact, Rosalind.
Therefore, it is both David's awareness of self and ability to make conscious decisions for the preservation of himself and others that reveal his maturity. The reader is also made aware that six years have passed.