The metre and form of the poem is written in a free verse that is influenced by iambic pentameter. You can notice the influence by the way 18 out of 32 lines have ten beats.
Because it is not in stanzas, the first thing you notice is the rhyme scheme of "Nearing Forty". Using this rhyme scheme, you can tell which lines belong together and where thoughts and ideas change. The rhyme scheme is:abacbcddeeffgghhiijjkbklcmjmcbcj Line 24, the l, line is the only one that doesn't rhyme with another line.
As for the form and punctuation of "Nearing Forty", there only appears to be just one capital letter in the whole poem, the first one (I in Insomniac). Through this, I believe Wolcott is trying to show that the poem was writing by an insomniac whose thoughts and words just seem to blend together. Similar to the way everything blurs together after a night of not having much sleep. This point is also emphasized by the way that there is only one traditional ending (period, question mark, etc.) in the poem. It is a period located after the last word in the last line. Either a semi-colon or a comma initiates every other change in thought or idea. There is also an epigraph written to Samuel Johnson before the poem starts to prepare the reader for the restless and endless ideas that are to come
As a whole, I feel as if "Nearing Forty" should be the anthem for every middle-aged man who's questions and ideas about life, death, future, past, and self-importance keep them awake at night.