And nothing I cared, at my sky blue trades, that time allows In all his tuneful turning so few and such morning songs (42-43)
The key here is the word turning. What he thought was an endless childhood has become "so few and such morning songs." Things have changed, and looking back, the speaker realizes now that he's older that his days were limited. Also, he uses the word "morning" but he could also be alluding to "mourning," as in, his poem is a "mourning song" about youth. He misses how things used to be and there's nothing he can do now. Too bad, so sad.
Time held me green and dying Though I sang in my chains like the sea (53-54)
At the end of the poem, the speaker's perspective on youth has shifted in a major way. He's changed now that he's older and he describes himself as "dying." Earlier in the poem, youth on the farm was full of life, but now it's the opposite. He sees it as a time of dying, even though back then, he felt very much alive. It's a sort of a metaphorical death. He wasn't actually, physically dying, but his youth was dying, and what's worse, he didn't realize it until too late. Finally, in the last line of the poem, the speaker puts two opposite things next to each other, possibly to suggest the paradoxical nature of youth seeming both eternal and too brief. "Chains" suggest imprisonment, while the "sea" is fluid and in motion and vast. Normally, we don't think of the sea as chained but as something free to move however it wants. One effect of placing these two words next to each other is to suggest a feeling of being both trapped and changing through time.