His point is that adults, who have been absorbed into the “group think” of society, are unable to think for themselves. Emerson says, famously, elsewhere in this essay that “Whoso would be a man must be a nonconformist.” What he means, is that the effect of society is to erode one’s ability to think independently—as he puts it, “Society everywhere is in conspiracy against the manhood of every one of its members,” in that the desire to live a life of ease, to conform, to go along with the common thinking makes people intellectually and morally lazy. This kind of mental sloth is a kind of “jail,” in that we rely on others to think for us, rather than relying on ourselves.
Emerson is also making a comparison between the mental activity and judgement of boys and men. His point is that boys, unencumbered by adult cares, have a freedom of judgement and an intellectual curiosity that people lose as they get older. “The nonchalance of boys who are sure of a dinner, and would disdain as much as a lord to do or say aught to conciliate one, is the healthy attitude of human nature,” Emerson writes. In other words, the “nonchalance” or disregard boys feel for what others might think of them, affords them the independence of thought and action that Emerson prizes most.
In "Self-Reliance" Emerson notes that a baby can control an entire room with its spontaneous and "uncontolled" behavior. As we grow older and come to realize that society places demands on us, demands about how we are to behave and think if we are to be "good members" of it. "For non-conformity, the world whips you with its displeasure." If we chose not to live outside the mold, we lose more and more of our freedom --- in a matter of speech, we are "jailed."
This is a common theme in Romanticism. In "My Heart Leaps Up." Wordsworth notes that "the child is father to the man." In the "Intimations of Immortality" he goes further to say that, "Our birth is but a sleep and a forgetting."
It sounds to me like Emerson is saying that as a person matures and becomes an adult, they become more tied down because of what they know - their consciousness. A child can daydream and imagine to his/her heart's content and believe that all things are truly possible. An adult knows better because as a person grows up, they realize the limitations that exist in life and that happy endings aren't a guarantee like they are in fairy tales.