I think Jack learns that being "grown-up" is not all fun and games. As children, the idea of being an adult brings with it dreams of freedom to do whatever one wants whenever one wants, but Jack learns that these are just dreams and not reality.
For example, when he pulls the other boys away from the fire they are using to produce smoke (in the hope a passing ship will see it and they will be rescued), they lose their opportunity to signal a ship that does actually pass. He wants the boys to help with the hunt so they'd have enough to make a full circle around their prey. He realizes that he is in the wrong and that he has made a big mistake, as he has failed to anticipate the possible consequences arising from his decision.
Being an adult means that one must keep several irons in the fire, so to speak; one must keep one's eye on a number of responsibilities, and one is not simply free to pick and choose what one wants. Jack learns that adults have many responsibilities that get in the way...
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