I’m a little surprised no one has challenged this statement. Keaton’s career faded, but not because his talent dried up. In 1928 he made the tragic mistake of signing with MGM, thus giving up control of his films to unimaginative studio heads. Unable to create films up to his own standards of quality, and forced to take a backseat to the likes of Red Skelton, he fell into alcoholism and other personal problems from which it took him decades to recover.
Unfortunately, his last feature film appearance, in A Funny Thing Happened on the Way to the Forum, doesn’t do much to restore his reputation. But if you’ve ever seen an appearance he made on Candid Camera in the late '50s or early '60s, you know he was as hilarious in his 60s as he was in his 20s.
The setup was so simple only a genius of Keaton’s magnitude could have made it work. He sat at a lunch counter and, while the people next to him looked on (not knowing who he was, of course), fumbled with his utensils and the salt and pepper shakers in ways that, while I can’t do them justice, were, trust me, hysterical. At one point, his toupee falls into his soup, and he picks it up, wrings it out, and puts it back on his head. The people next to him thought they were seeing some poor bumbling fool and could barely keep a straight face. It’s been years, perhaps decades, since I’ve seen this clip, but my memory of its brilliance is still clear.
The last film he directed was a short called The Railrodder, made in 1965, just a year before he died. It is a little gem and, IMHO, further proof that Buster Keaton was, like Groucho, Fields, Burns, and others mentioned previously, a worthy counterexample to the OP. Sadly, there just isn’t as much evidence of the fact as in the other cases.