In "My Heart Leaps Up," the speaker of the poem sees his life as existing in 3 stages.What are these different stages?

In Lines 3-5 of "My Heart Leaps Up" William Wordsworth sees his life in three parts, as a young child ( "when my life began"), as a man ( "So is now I am a man) and as an old man "( So be it when I shall grow old") . As he says in the poem, he hopes he will always see nature as something beautiful and can retain that mystical wonder when his "heart leaps up" when he sees a rainbow, or something beautiful in a natural setting.