Prior to their being attacked by the Socs, Ponyboy Curtis and Johnny Cade sometimes exhibit typically adolescent behavior (especially Ponyboy). However, they are forced to grow up quickly in some ways after Bob's death. Johnny's reaction to Ponyboy's recitation of the poem in Chapter Five is an excellent indicator of his maturity.
In Johnny's response to the poem, it is evident to the reader that he has changed and began to notice see the beauty in life that he failed to recognize before their time at the church.
Johnny was staring at me. "Where'd you learn that? That was what I meant."
"Robert Frost wrote it. He meant more to it that I'm gettin', though...I always remembered it because I never quite got what he meant by it."
"You know," Johnny said slowly, "I never noticed colors and clouds and stuff until you kept reminding me about them. It seems like they were never there before."
By admitting that he does not understand Frost's message, Ponyboy makes it clear to the reader that he has not reached the same level of maturity as Johnny. However, Ponyboy does experience extreme "growing pains" after the deaths of Johnny and Dally, as well as other events.