One good poem for a 40th birthday is "Life begins at 40" by Martin Dejnicki . Another good poem is "The Five Stages of Forty" by Mark Shaughnessy.
"Life Begins at 40" by Martin Dejnicki is a poem that focuses on the special benefits of someone becoming 40. Specifically, the poem praises the individual's ability to grow older without fear, to exhibit sound judgement, and to stand upright throughout the changing seasons. The poem climaxes by revealing the metaphor that the individual turning 40 is being compared to a tree. This helps the poet imagine turning 40 as a special event because, like trees, younger individuals use the aging person to reach greater heights and rely on him as a steady, unchanging force. And because 40 is a young age for a tree, the poem helps the birthday celebrant to feel younger.
"The Five Stages of Forty" by Mark Shaughnessy is a poem that imagines a trip to Las Vegas as a transformative journey for someone about to turn 40 years old. The speaker spends much of the trip dreading aging and desperately seeking to hold on to remaining youth through both self-delusions and products such as wrinkle cream and Botox. The speaker eventually encounters a much older individual who is peaceful and serene, not worried about mortality at all. This provides the inspiration to embrace present life instead of worrying what the future may hold.