Final answer:
Mr. Raymond is suggesting that as children grow older, they become less visibly emotional about injustices, even though they may still recognize them as wrong.
Step-by-step explanation:
When Mr. Raymond tells Scout, "Things haven't caught up with that one's instincts yet... Let him get a little older and he won't get sick and cry. Maybe things will strike him as being—not quite right, say, but he won't cry, not when he gets a few years on him," he is implying that as children grow older, they often lose the intense emotional response to unfairness and instead develop a more numbed reaction to the injustices of the world. While a younger child might react to unfairness with visceral upset, such as getting sick or crying, an older child or adult might still recognize something as unjust but without the same emotional response. This speaks to the way societal norms and expectations can dull one's instinctive reactions to wrongs as they age.