Lines 3-5 are an example of anaphora, a device where sucessive phrases begin with the same or similar words:
So was it when my life began, / So is it now I am a man, / So be it when I shall grow old / Or let me die!
Subsequently, line six is an example of hyperbole, or an overstatment that emphasizes an idea but is not meant to be taken literally.
Finally, in line 7 we see a paradoxical statement: "The child is the father of the man". This statement appears to contradict itself until the reader realizes that Wordsworth is speaking figuratively and not literally.