The symbolism in "As I Grew Older" is robust and center-stage in the poem. Indeed, the poem is structured entirely around two key images, which are repeated for emphasis.
The first symbol is that of the speaker's "dream," which appears "bright like a sun." The dream is symbolically associated with brightness ("the light of my dream") and draws upon the fact that "light" in Western literature and culture tends to represent the positive and the uplifting.
The second symbol, that of the wall which "rose" between the speaker and his dream, is accordingly associated with "shadow" and darkness. The "thick wall" casts a shadow and blocks out the light of the dream. Notably, the speaker declares, "I am black," drawing an association between his own blackness and the fact that he is forced to "lie down in the shadow" rather than pursue his dreams. This forces the reader to interrogate the symbolic associations of blackness with negative elements, and lightness, or whiteness, with the positive.
Ultimately, the speaker refuses to accept these associations. It is with his "dark hands" that he breaks through the wall that has separated him from his dreams. His own blackness will not hold him back from smashing "this night," breaking "this shadow / Into a thousand lights of sun." In using the imagery of "dark hands" dismantling the wall of shadow that holds him back from his dream, Hughes is questioning and subverting the lazy use of symbolism wherein all that is black is bad.