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.