The older boys that Jerry see at the wild beach play an important role in terms of firstly making Jerry aware that he is still a child and secondly showing him what he needs to do in order to grow up and become a man.
If we look at the story, when Jerry first goes to the wild beach, he greets the boys with "desperate, nervous supplication," wanting to fit in. Jerry views them as being "men," which furhter underscores the difference between himself and them. When, however, they dive through the tunnel and Jerry is unable to follow them, and Jerry responds by being silly and trying to make them laugh, note how they respond:
They looked down gravely, frowning. He knew the frown. At moments of failure, when he clowned to claim his mother's attention, it was with just this grave, embarrassed inspection that she rewarded him.
The boys thus highlight to Jerry how young he still is and reinforce his childlike state. They also, through the tunnel that they dive through, present him with the way of becoming "men" like they are and the means of how he can achieve his rite of passage and grow up.