Johnny definitely grows up throughout the course of the book. Even as the book opens, he is a pretty impressive young lad. He is a talented silversmith and mature for his age. When Johnny hurts his hand, he is forced to grow up fast. No longer is his life mapped out, because he can’t be an apprentice any more. Johnny is homeless for a while, and hungry. He has to use his wits to survive.
After he attains gainful employment with the newspaper and the revolutionaries, he continues to return to Cilla, after he has been on his own and has the influence of the Sons of Liberty such as John Adams, John Hancock and Paul Revere, he grows apart from her. He begins to look for more out of life. Cilla is the same, but he is more mature and has a higher calling.
A final way that Johnny matures is that by the end of the book he no longer romanticizes war. He has seen what battle can do, and while he still believes in the cause he takes a realistic view of the fight.