Yeats’s speaker looks back with quiet dismay at the difference between body and heart. He once believed youth could be prolonged by physical discipline—dumbbells and the foil—as if keeping the body fit would keep the whole self young. But he discovers that the emotional core has its own timeline: desire may remain, yet the heart no longer burns with the intensity it once had. In the second stanza, he can still talk and charm, but he no longer feels the old faintness, the breathless excitement of being beside a woman. The repeated question “Oh, who could have foretold / That the heart grows old?” gives the poem its rueful, almost baffled mood. It is not loud grief; it is the melancholy surprise of someone who prepared for physical age but forgot that feeling itself could tire.
A Song — Summary
I thought no more was needed Youth to prolong Than dumb-bell and foil To keep the body young. Oh, who could have foretold That the heart grows old? Though I have many words, What woman's satisfied, I am no longer faint Because at her side? Oh, who could have foretold That the heart grows old? I have not lost desire But the heart that I had, I thought 'twould burn my body Laid on the death-bed. But who could have foretold That the heart grows old?