In stanza five of Wordsworth's "Ode: Intimations of Immortality," "the narrator explains how humans start in an ideal world that slowly fades into a shadowy life..."
The poet introduces his idea that we dwell in heaven before we are born and joined with our bodies; as infants, we retain a strong sense or memory of what heaven is like.
But trailing clouds of glory do we come
From God, who is our home:
Heaven lies about us in our infancy!
However, as a "growing boy..."
Shades of the prison-house begin to close...
But He beholds the light, and whence it flows,
He sees it in his joy...
As the young boy begins to grown, the "prison house," a disconnection between our souls prior to birth, which has remained in infancy, is beginning to fade—though the young boy is still just close enough to heaven to see the light, know where it is coming from, and find joy in the world. Wordsworth infers that the young boy is approaching a precipice, almost ready to move into unknown and dark territories, where heaven's light will be no longer visible, and will be forgotten.
In childhood, according to Wordsworth, one’s own immortality is intuited and so young people are perpetually joyful; they have a “heart of May” not because their bodies are strong and capable but because of their spiritual health.
Wordsworth makes his argument that what makes young people capable is not their physical health, but their spiritual well-being: close at hand because they are still young and still connected to the source of their soul's initial joy: heaven.
The youth moves even farther from heaven, closer to the "precipice," but still a glimmer of heaven remains on the fringes of his awareness: though he is growing up and growing away from "spiritual health," he is "still Nature's priest." As a early Romantic poet, one of the elements in life Wordsworth felt was so important was one's connection to nature and the natural world.
As the youth becomes a Man, his connection to his "early form" is lost in the realities of the world: its obligations, work and trials. Nothing is left of that joyful sense that had earlier tied the Man to heaven as a baby. He loses hope as he plods along, continually without release, through each "common day."