Though "My Heart Leaps Up When I Behold" might seem to be just a happy poem about nature, it also touches on the topic of the passing of time, mostly in the form of aging. We hear about the speaker's entire lifespan in this poem, from his birth to his conjectures about his death. All along the way, the speaker wants to maintain his love of nature, which he had as a child. In a sense, it's like he's some kind of super-awesome time traveler. Even though he's getting older, he's still young in his appreciation of nature. Our speaker has some pretty interesting ideas about the influence of childhood on the rest of one's life, and he places a big emphasis on staying young at heart.
Good news, bad news: this poem shows how joy in nature can endure, even though life is fleeting.
This poem stresses that childhood experiences play an important role in forming adults. That's kind of why them call them formative experiences, right?