How do Chapters One to Ten of The Chrysalids show that David is changing into an adult?
There are two events in particular that demonstrate how David is having to grow up very quickly in his world. We see that through his friendship with Sophie and then his Uncle Axel's discovery of David's "talent" and the existence of his friends, that David becomes quickly aware of the dangerous world in which he is growing up and the necessity of keeping various secrets for his own safety and for the safety of others as well. Note what David says after Uncle Axel warns him to keep his talent a secret:
But now, out of Uncle Axel's anxious insistence on my promise, the feeling of a threat was strengthened. It was still shapeless to us, but it was mor real. Furthermore, in trying to convey Uncle Axel's seriousness to them I must have stirred up an uneasiness that was in all their minds, for there was no dissent.
Thus David becomes aware of how dangerous their world is, and the way in which the discovery of his gift would constitute a "shapeless" threat or danger. This is of course highlighted when Sophie's "mutation" is discovered and she, along with her parents, are captured. At the end of Chapter Five, there is a very poignant moment as David becomes overwhelmed by "bitterness, self-contempt, and abasement." As he clutches the ribbon and curl of Sophie he sobs out his pain, which is of course intensified by the realisation that his father and the authorities would do exactly the same thing to him if they knew about his gift. These two events, therefore, demonstrate how quickly David needs to grow up and become an adult, taking very seriously the situation in which he finds himself.