This poem covers the range of human life, from childhood, to adulthood, to old age and death. It stresses the influence of childhood throughout life, not just until one "matures." The most important part of that childhood influence, for our speaker, is the unbridled joy that a child finds in the natural world.
Line 3: This line describes the speaker's childhood. It feels a little nostalgic, longing for the past.
Line 4: This line establishes constancy from childhood to manhood: the speaker is still enamored of rainbows, just as when he was as a child.
Line 5: Here we switch from past to future—the speaker hopes to feel the same joy when he's old. Even though a lot may change as he ages, one constant will always be that his heart leaps up at the sight of rainbows. Note that three lines in a row have started with the word "so," which is an example of anaphora, the literary term for multiple lines starting with the same word or phrase.
Line 6: The poem touches on the concept of death here. For the speaker, the moment he stops being excited by the sight of rainbows is the moment he's effectively stopped living.
Line 7: This line is an example of a paradox, a seemingly contradictory statement that holds a hidden truth. The idea that a child could be father of a man makes no sense, until you think about how every man was once a child, and grew out of this child. Far out, man.
Lines 8-9: The speaker is asserting that, no matter what else happens, he wishes to have a respect for and admiration of nature every day of his life, no matter his age.