A man on a train notices a woman smiling at him, and not just once. He reads her smiles, her glances and even the way she gets to her feet as signs of interest. The poem is a small joke built on his misreading, and the tidy rhyming couplets help it along, pushing him confidently toward a wrong conclusion.
At first the speaker is delighted. He congratulates himself with "you've still got it man" and imagines he has "a fan." The exclamation marks and the quick, chatty rhythm capture his pleased excitement. Then the final couplet flips everything: she was only offering him her seat. The title, "Age Concern," gives the game away in hindsight, since she took him for someone older who needed to sit down.
The mood is light and comic, with a touch of gentle embarrassment. The speaker's vanity gets punctured, but the poem laughs with him rather than cruelly at him, and he seems to be telling the story against himself.