Yes. The speaker has always been courageous. When speaking of an "unconquerable soul," he is already made it through difficulties. Thus, it gives the idea he has always had the courage, and it's not only in the present but also in the past. Another instance in which the speaker shows he has always been courageous is when talking about the "menace of the years." He says it "finds, and shall find me unafraid." This implies he has always been courageous. The present moment "finds" him unafraid.
Additionally, the following lines refer to a time in the past and depict the courage he had then: "In the fell clutch of circumstance/I have not winced nor cried aloud"