After Sammy walks out of the A & P and realizes the girls are long gone, he sees the store's manager, Lengle, taking his place at the check out counter, and Sammy's first thought is "my stomach kind of fell as I felt how hard the world was going to be to me hereafter." This quote epitomizes the "coming-of-age" experience.
Admittedly, Sammy is first drawn to the girls, especially "Queenie," because his teenage hormones control his behavior, but there is a point at which he actually feels sorry for the girls. When Lengle, the manager, challenges the girls for wearing only bathing suits into the store, Sammy notices that "Queenie's blush is no sunburn now," and in a sort of recompense for Lengle's un-gentlemanly behavior, Sammy says, "'I quit,' to Lengle quick enough for them to hear, hoping they'll stop and watch me, their unsuspected hero."
The girls, of course, do not hear this chivalrous behavior because they are on their way out the door. And when Sammy finally leaves the store and realizes the girls are nowhere in sight, he realizes his gesture has been in vain. The questions for the reader at this point are whether or not Sammy regrets quitting his job and whether this event is truly a "coming-of-age" moment.
I think the answer is a qualified "no" to the first and an emphatic "yes" to the second. Does he regret quitting his job? Yes, for a moment, but, ultimately, no--throughout the story, we see that Sammy has achieved a rather cynical detachment from both his job and the store's patrons, whom he often refers to as sheep. One could argue that Sammy has already "moved on" intellectually from this job and is ready for more sophisticated challenges. Does he truly come of age? Sammy's realization that life is going to be hard is a very mature acknowledgement that taking a stand on principles (whether hormone or morally driven) will complicate his life. The fact that he comes to this conclusion is evidence that he has transcended his teenage self-absorption and understands that consequences flow from certain actions. In other words, the protections accorded to immature teenagers by adults no longer apply to Sammy: he has irrevocably challenged an adult authority figure and indicated his willingness to accept the consequences--in a very real sense, he has come of age.