If the speaker of "Fern Hill" made one mistake as a youth, it's that he failed to realize that being young doesn't last forever. So at the beginning of the poem, we've got a footloose and fancy free youth, but by the end, we're left with an older, wiser, sadder speaker. That change occurs gradually and subtly, but if you look closely at the imagery of the poem, you might be surprised to find you'd seen it coming all along.
Although the speaker in "Fern Hill" acted as if his youth would last forever, images of motion, such as streams and rivers, create a tension that foreshadows the eventual passing of youth.
"Fern Hill" is a poem that uses the natural cycle of a day from light to dark as a framework for the passing of time, which both frees and imprisons the speaker.