You're only young once, and all those other clichés. For the speaker of "Fern Hill," youth is everything it should be—joyful, carefree, and oh so fleeting. And that's kind of the problem. It's easy for our speaker to feel the wonders of youth as everlasting, but all along he was doomed to be yanked irreversibly into adulthood, just like the rest of us. It's the bummer of all bummers.
All good things must come to an end, right? But for this speaker, the pastoral setting contains his joy and innocence as a youngster and becomes the epitome of how the speaker felt about youth.
Through several religious references, the speaker assigns a divine innocence to his youth, which is later tainted by the passing of time.