It is an exasperating fact of life that the older you get, the younger the young get. Its corollary is that the greater your own age, the greater the age beside the word "young" in your internal dictionary. You won't really notice the latter until one day when you're talking to a guy with three kids and a second mortgage, you will hear yourself saying something like, "you're just a kid. Give it time..." You will, I guarantee, want to stick your tongue in the nearest wall socket, but deep down you will actually believe what you are saying.
"If I knew I was going to live this long, I'd have taken better care of myself."
It is a moth-eaten cliché that dying young is the best business decision a poet can make. There is just something about ending with an ellipsis rather than a period. The unfinished career has a maddening allure--akin, I think, to finding the last pages of a mystery ripped out. You may curse the vandal and his progeny's progeny, but you will always remember exactly where the plot stood when it was cut off. In any case, it is hard to argue that shucking this mortal coil when one is firing on all cylinders artistically is preferable to a slow public decline.
Fortunately it is not an either/or proposition. Below are some poets who continued to grow, innovate and --ahem-- mature as artists while many of their age-mates were resigning themselves to daytime TV and obsessive contemplation of their lower intestinal tracts.
While it is hard (not to mention unwise) to generalize, it seems to me that much late-career work tends to use a scaled down palette, giving the work a more universal, less solipsistic feel. Often a certain snakiness gives way to a pointed essentialism--an impatience with, if not outright distrust of, show. I guess it can be argued that this is evidence of diminishing creative powers, and in some cases it probably is, but it seems to me to be too common, too intentional, and too artistically valid to be only that. Horizons seem to both broaden and narrow tangibly late in writers' careers. As readers, we feel the end of day coming, but there is always that question of whether the sun is falling or the world is rising to meet it.
Selected Poems  [expanded edition] / A. R. Ammons
The Really Short Poems of A. R. Ammons  /
The Collected Poems 1945-1975  / Robert Creeley 
Memory Gardens / Robert Creeley
Selected Poems 1945-2005  / Robert Creeley
Poems Seven: New and Complete Poetry  / Alan Dugan
New Collected Poems  / Robert Graves 
The Collected Poems  / Stanley Kunitz
The Collected Poems: 1931-1987  / Czeslaw Milosz 
New and Collected Poems: 1931-2001  / Czeslaw Milosz
The Complete Poems  / Marianne Moore 
Atlas of the Difficult World: Poems, 1988-1991  / Adrienne Rich 
The Fact of a Doorframe: Poems Selected and New, 1950-2001  / Adrienne Rich
Adrienne Rich  [CD]
Everything is Burning  / Gerald Stern
This Time: New and Selected Poems  / Gerald Stern
The Collected Poems: 1943-2001  / Richard Wilbur 
The word geezer in the title is not used with derogatory intent. My dictionary defines it as "an old person, especially an eccentric old man." I, for one, am a geezer-in-training and hope someday to go pro.
As always, comments, corrections, disagreements and suggestions for future posts are greatly appreciated.