[…] I was prince of the apple towns And once below a time I lordly had the trees and leaves (5-6)
So, the speaker doesn't come out and say, "I was young, once." But, he chooses to describe himself as "prince" and says, "once below a time." The figurative language here immediately sets the scene—he's young, he feels invincible, and he's got it made… for now.
Time let me play and be Golden in the mercy of his means
And green and golden I was huntsman and herdsman, the calves
Sang to my horn, the foxes on the hills barked clear and cold (13-16)
Again, the speaker doesn't use the word youth, but by now we know he's describing what his youth felt like on the farm. It was a "golden" time and again, he seems to be in charge, as if youth has anointed him dominion over the landscape.
[…] it was air And playing, lovely and watery And fire green as grass. (20-22)
In stanza three, the speaker continues to praise youth. The "it" we think is youth, and the speaker leaves no doubt that he thought it was "lovely." Plus, the speaker's using nature imagery to describe youth, as if it were something as natural as the setting where he finds himself. This seems important because it expresses the speaker's belief that youth was something that shaped and was shaped by the landscape around him. Youth tied him to the setting and the speaker loved every inch of it.
Before the children green and golden Follow him out of grace. (44-45)
Here, youth is expressed as children green and golden. Although it's sin that casts man and woman out of Eden, in this poem, it's time that leads children out of youth. So, for the speaker, youth is a kind of innocence, which is lost as the speaker grows older. In other words, youth sounds like perfection, and everything else is just… not.
Nothing I cared, in the lamb white days, that time would take me (46)
The speaker says, "the lamb white days," which suggests innocence and purity. On the other hand, associating youth with "lamb" could allude to the idea of a lamb being led to slaughter. Just as time is leading youth away, so is the innocence of youth being destroyed by time. What was originally a time of great confidence has begun to shift to a time of vulnerability and naïveté.
Oh I was young and easy in the mercy of his means, Time held me green and dying Though I sang in my chains like the sea. (52-54)
The speaker finally directly says, "I was young." We know this by now, but it's almost like a confession of sorts. Whereas earlier in the poem, youth is referenced in figurative language, here the speaker says directly that the past was his youth. While he was living it, youth was a magical time. But now, at the end of the poem, the speaker sees that time with more clarity.
The final two lines are a reference to youth as well, but with a different tone than the rest of the poem. Here, the speaker is no longer free, but "in chains like the sea." From the viewpoint of an older, wiser person, he sees his youth as a time mixed with good and bad, a bittersweet golden age that would inevitably pass.