One of the refreshing aspects of this forum is the evidence of close listening that informs so many of the comments. In contrast, much discussion of classical music in some online forums smacks of little more than received opinion. In this spirit, I'd like to encourage friends to try to ignore biography when listening to late music, reading late writings, viewing late paintings, and so on.
One needn't be aged to have one's late works dismissed as evidence of decline. Criticism during the first third of the 20th century often repeated the truism that Chopin's late nocturnes and mazurkas are products of enervation, and his cello sonata was almost entirely ignored. More recently, an echo chamber consigned Schumann's late chamber works, concertos, and choral works to the realm of the uninteresting, even though the composer was vigorously reinventing his compositional technique in them.
For longer lived composers, particularly those 'romantics' who wanted their music to be 'about something' more than sonic surfaces, received opinions have been no less misleading. In the case of Bax, he brought it upon himself with his own comments, perhaps. But while no orchestral work after Symphony 7 matches the earlier ones in invention and imagination, his cello sonata, piano trio, and a handful of other works from the 1940s display unflagging musical powers.
Many members of this forum surely recall the disdain in which all Rachmaninov's works after the Great War -- save the Rhapsody -- were long held.
The case of Roy Harris, where decades of writers echoed the scurrilous claim that his wife Johanna composed the later works, is particularly unfortunate. Symphonies 11 and 12 stand in relationship to the earlier ones much as Vaughan Williams's Symphonies 8 and 9 stand to his earlier works. VW is the greater composer, to be sure, but Harris stands tall as well and is clearly pursuing different ends in the late works.
Few composers reinvent themselves as thoroughly as did Stravinsky and Janacek. That does not mean, however, that other late composers are attempting to repeat their earlier successes. Unfortunately, listeners confronted with a still recognizable style are all too often disappointed when the works do not go where earlier ones went. Thus begin all too many stories of decline.
There is no question that as their bodies decline, so, generally, do many composers' powers of concentration. Yet there's also no question that age brings new perspectives that require a different, perhaps subtler means of expression. If it will help us follow music that proceeds cogently to unfamiliar places, I suggest we err on the side of biographical ignorance when listening to late works by composers.