How we cite our quotes:
[…] that time allows
In all his tuneful turning so few and such morning songs (42-43)
Oops, sounds like something's changed here. Time is still personified as a him, and this time the speaker is concerned with "his tuneful turning so few and such morning song." What's up with that, time? What was a merciful bounty has become something that "turns" and only has a "few" morning songs. The speaker is beginning to experience the passing of time and it's not like he thought it would be.
This is also the section of "Fern Hill" that might allude to the Pied Piper of Hamelin. The Pied Piper played a flute that led children away from their town. He was owed a debt and the town didn't pay, so he took the children as payment. In the same way, time is playing a tune that leads children away from youth. It lends a creepy note to a so far joyful poem.
[…] that time would take me
Up to the swallow thronged loft by the shadow of my hand (46-47)
Throughout the whole poem, everything has been shining and light and golden, and time has been on the speaker's side. But not in this last stanza. Here time has taken him by the wrist and begun to lead him away. Suddenly, we don't see the speaker in light, but as shadow himself. And time's no longer the passive, watchful figure that he was at the beginning of the poem. Here, he's acting directly on the speaker.
I should hear him fly with the high fields
And wake to the farm forever fled from the childless land. (50-51)
Time is folding up all the pastoral beauty the speaker loved and skipping town in the middle of the night. What's important here is that the speaker doesn't notice until it's too late. It's not that he watched as time packed up and drove away. Rather, he wakes and all that's left is the "childless land."