There are many themes explored in this iconic poem. Perhaps the most prominent ones are of regret and a sense of lost youth. There is also a sense of lost or unrequited love, as when the narrator frequently speaks to an unknown "you" as a companion. The idea of lost youth and regret over the past is repeated throughout the poem: for example in the segment which repeats "there will be time", "for a hundred indecisions" and the idea of "a hundred visions and revisions" there is a sense of having unlimited time to relive the past wishing to have a different outcome.
The theme of lost youth is evident in many descriptions of physical aging (his thinning hair, bald spot and thin arms) and the yearning for imaginative and romantic visions ("I have heard the mermaids singing...I do not think that they will sing to me") that seem to be a fragment of memory from a lost past.