How can the themes of rebellion and immaturity be used in a thesis statement for "A & P"?

One possible example of a thesis
statement:

In the story "A & P," John Updike
presents the growing pains of adolescence. Therefore, the main character displays the
typical characteristics of teenage behavior such as rebellion, immaturity, and the need
for individuality.

Focus on the theme of "growing up"
through three distinct behavioral attitudes.  Sammy's attitude (in his first-person
storytelling) is very distinct.  He comes across as someone who is attempting to tell a
story with apathy, but it is clear that he wants people to take him seriously.  The main
attitude he portrays, to me, is one of a teenager trying very hard to
sound like an adult.  Therefore, I'm not sure that your thesis
should only focus on Sammy.

The thesis suggestion above
clarifies your ideas. However, to focus on "rebellion," "immaturity," and the "need for
individuality" seems to point as much or even more to the girls who enter the store in
their bathing suits. I might tweak it to say something
like

In the
story "A & P," John Updike sums up the growing pains of adolescence in just one
afternoon.  Through the choices characters make and the reactions of others to those
choices, the author shows that growing up is a mixture of rebellion, immaturity, and the
desire for
individuality.

This way, you
can focus on each of the three attitudes you originally chose, but instead of limiting
yourself to Sammy's actions, you can choose from the actions of virtually anyone in the
store and even Sammy's family (who reacts to his quitting as "the sad part").  This
allows you to stay with your original idea but provides more examples as
proof.