In “Through the Tunnel,” how does Jerry show growing maturity as he prepares for the swim through the tunnel?

Once Jerry sets his mind to swimming through the tunnel like he saw the big boys do, he practices holding his breath and staying under water day after day.  This kind of meticulous practice provides evidence of his growing maturity.  After several days of going to the wild bay to practice, "He did not ask for permission [...] to go to his beach."  Children ask permission and wait for adults to grant it; however, Jerry begins to exercise his new maturity and independence by acting without asking his mother -- as he has in the past -- if he can go to his own beach and not accompany her to their old, "safe beach." 
Further, Jerry is developing the ability to delay gratification of his wishes.  Earlier, he couldn't wait for his mother to purchase the goggles he needed, and he pestered her until she took him, and then snatched them from her hand, like a child.  But now, he thinks, "if he tried, he could [probably] get through the long tunnel, but he was not going to try yet.  A curious, most unchildlike persistence, a controlled impatience, made him wait."  Jerry's growing maturity makes it possible for him to delay gratification of his desires, and he waits to attempt the feat until he is more sure of his ability.