When did Jonas first lie to his parents in The Giver?

Jonas lies to his parents in the end of Chapter 16, after they ask him if he understands that love is a vague concept.

No one in the community is allowed to lie, as far as Jonas knows.  Precision of language is very important.

He had been trained since earliest childhood, since his earliest learning of language, never to lie. It was an integral part of the learning of precise speech. (Ch. 9)

The community values Sameness, and the entire goal of his civilization seems to be avoiding discomfort.  They want to ensure that everyone is toeing the line.  That means that children are indoctrinated from a young age to the community’s procedures and expectations.

When Jonas is selected for the prestigious assignment of Receiver of Memory, one of his instructions is that he can lie.  This confuses and horrifies him.  It never occurred to him that anyone in his community might lie.  He wonders if anyone else has the same instruction, and begins to doubt from that point on if anyone in his community is telling the truth.

This is a pivotal moment in Jonas’s character development.  It is the first time when he begins to wonder about his community.  Jonas will soon learn that his entire community is a lie, in a way.  It professes to be perfect and orderly, with content citizens and no negatives.  Jonas soon learns that the community has carefully constructed this illusion by keeping its citizens almost completely cut off from the human experience.

No one in the community feels any emotion at all.  They are carefully trained from birth to avoid attachment.  Jonas learns through the memories that attachment is part of being human. He realizes that his people do not experience humanity because they are cut off from love and joy as well as suffering.

When Jonas experiences the concept of love, he has already seen many memories of things that the community has stamped out.  The concept of family bothers him the most though.  He enjoys the connectedness that he feels in the memory, and he realizes that it is important.  In an effort to see if any of that connectedness is there, Jonas asks his parents if they love him.  He knows that he is not likely to get expressions of love back, but is totally unprepared for confusion and amusement.

His parents both explain to him that “love” is an imprecise word that has come to be meaningless.

Jonas stared at them. Meaningless? He had never before felt anything as meaningful as the memory.

"And of course our community can't function smoothly if people don't use precise language. You could ask, 'Do you enjoy me?' The answer is 'Yes,' " his mother said. (Ch. 16, p. 160)

When Jonas’s parents ask him if he understands, he lies and says he does.  He lies because he knows that they are the ones who do not understand.  Since no one in the community can experience real affection, or attachment, there is no way that they can understand love.  His entire community has been carefully constructed to avoid it.

While different versions of the book will have different page numbers, you should easily be able to find this incident because it is near the end of Chapter 16.  In my edition of the book, it can be found on page 160.