In Chapter 5 of the Chrysalids, David's character is described more completely. He is now at the crucial stage of boy-man, having characteristics...

David’s secret with Sophie demonstrates how he is a child, but his protectiveness of her also shows his maturity.

Sophie is a double-edged sword for David.  He is young enough to still be naïve about his friendship with her.  He does not understand the risks that he is talking.  Sophie and her family understand and accept those risks, but David does not.  It never occurs to him that he might be in a position where he would be asked to betray Sophie.

When Alan sees Sophie, David’s first reaction is to fight him.  This is a rather immature response on some levels, but also a chivalrous one.  He is trying to protect her.

My intention was to gain a few minutes for Sophie to put her shoes on and hide; if she had a little start, he would never be able to find her, as I knew from experience. (Ch. 5)

Unfortunately, David is in a tough spot.  His own father is one of the most tyrannically faithful followers of the True Image in Waknuk.  David should realize that there is no way to avoid his scrutiny or his wrath.

David wants to go with Sophie, demonstrating the immature side of him, but the Wenders understand that this is not a good idea and tell him to wait and go home.  David waits as the Wenders suggest, but when he gets home his father already knows too much.  David remains bravely mute, not telling them anything about Sophie.  When David’s father tells him to go to his room, he knows that his father will try to beat the truth out of him.

I hesitated. I knew well enough what that meant, but I knew, too, that with my father in his present mood it would happen whether I told or not. I set my jaw, and turned to go. (Ch. 5)

David tells his father what he knows, but feels wretched for it.  In holding out, he was trying to be grown-up.  His self-loathing also demonstrates some maturity.  David understands that he has betrayed Sophie, even though he did not mean to.  It is a very adult problem to have.

This incident is a turning point for David.  Everyone has an incident that causes the person to go from being a child to being an adult.  This was David’s.  Any innocence he might have had was lost when he gave Sophie up.