You smiled, you spoke, and I believed,-- Walter Savage Landor (1846)
By every word and smile deceived.
Another man would hope no more;
Nor hope I what I hoped before:
But let not this last wish be vain;
Deceive, deceive me once again!
Landor was in his sixties when he published that cute work, making him one of the very few poets I can think of who was worth a damn much past 50. There's Yeats, of course, and Hardy, who didn't start publishing poems until he was in his fifties, but had been writing them most of his life, just not publishing them. I'm not familiar enough with Hardy's work and life to know which of his poems were composed when, which makes assessing the quality of his later-written work difficult. Anyway, it seems there's some kind of creative spark that dies once someone gets into middle age. Eliot remarked on this, saying something to the effect that no one expects athletes to keep performing past 40, but people seem to expect poets to keep churning out masterpieces until six feet of earth finally silences them. But I wonder if it isn't something besides age.
Both Hardy and, in particular, Yeats, went through profound stylistic changes, with Hardy giving up novel writing in favor of poetry, and Yeats going through three distinct "periods" of writing style, each radically different from the other. Yeats also lived a full life from start to finish, getting involved with the Irish nationalist movement in his thirties, getting married for the first time and having children in his fifties. Landor himself switched between writing in Latin and English; he also lived a peripatetic life, moving around Europe like a vagabond, getting involved in different nationalist movements all the while. The idea is that the lifestyles of these men, full of adventures and changes, might have done something to keep them fresh enough to write decent stuff as old men. The traditional arc of a well known poet is to struggle in youth, then hit it big, and coast through a jaded life afterwards (see Wordsworth and Eliot as archetypes), or else to die too young to get jaded (the list is so long even those uninterested in poetry could name several). These three men were able to avoid that by embracing challenge and change.
Personally, I always wonder how anyone can do anything for twenty years and not get bored to death by it, bored to the point where you really don't put all your energies into it. When I need the services of a professional, I always look to see how long they've been doing the job, with my ideal time period being 10 years. That's enough time, I figure, for them to have mastered the field, but not so much time that they have become burned out hacks. The things you learn from poetry....