In lines 9-12, symbolism is applied through the image of fire. The “glowing” fire signifies the narrator’s dimming youth, as its dull embers will soon expire and turn to “ashes.”
In line 4, Shakespeare uses personification, "sweet birds sang". Birds cannot sing but he was telling us that the birds were chirping and it sounded like a song.
In line 6 and 7, Shakespeare uses personification again. He says, "As after sunset fadeth in the west; Which by and by black night doth take away". This is a personification because the night cannot "take away the sun.
All of these figurative language examples make the poem easier to read and visualize what Shakespeare is saying.