I just finished reading Ed McBain's "The Frumious Bandersnatch", and I'm a little sad. It's not a bad book, still better than a lot of what's out there, but it's nowhere near the quality of the old 87th Precinct books. There's only one crime, rather than the two interleaving and reflecting ones that he used to have, and, worse, I 'got it' about halfway through. He's been writing these books since the fifties, and it's starting to look like time to go. (Nice to see Ollie Weeks maybe falling in love, though).
Another one that was REALLY sad, was the last Dick Francis book. It read like someone had taken a Dick Francis plot outline, and just slapped together a book out of it (I understand that this is pretty well what happened, his wife mostly wrote the book). All of the life and sparkle, the 'Francis-ish-ness' was gone.
What do you think? Should authors let go and stop writing when they are starting to lose it? CAN they, when this is what they've been doing for the last sixty years?

I don't think the phenomenon applies only to elderly authors. There are some authors in their prime who have found a plot that works and keep rehashing the same thing over and over. The unfortunate part is that there still is a market for it. My example would be Danielle Steel. I used to read her, but then discovered that if you just changed the name of the heroine and hair color you would have the basically same book.
I think this can happen to any author regardless of genre. Terry Goodkind who writes the "Sword of Truth" fantasy series has become redundant and I don't read his books anymore. Every book it's the same rehashed plot (two main characters have traits that shouldn't work; but do. Ancient prophecy indicates that he will be betrayed; she betrays to save and all is forgiven). Even romance novels are the same. Take the Christine Feehan "Dark" series. How many ways can a Carpathian turn a woman?

It must be tough to stay fresh; especially as your popularity rises the public clamors for more of your books. How is it possible to churn out books in a manner that will keep your audience hooked and coming back for more? Personally if an author is good I will wait around and look for their next book. Look how long it took JK Rowling to write another "Harry Potter" book? Dennis Lehane books I love and I've caught up on them. When will he write another? I think his writing is gritty and captivating that I would wait around. I'd rather not let an author sacrifice his work all for the sake of the almighty dollar.

Dennis Lehane is changing and growing, I doubt if he will go back to his series characters. I'm astonished at how much better he is with every book.
You're right that an author doesn't have to get older for the rot to set in - look at Robert B. Parker. When you read the Spenser books of the last ten years or so, it's hard to believe how really good the first couple were.

I'm willing to wait for as long as it takes for the next installment of Harry Potter! Such a brilliant series!!

I used to read the Scarpetta series by Particia Cornwell, but the last few have left me very disappointed. It was an excellent series in the begining, now it seems to have fallen flat.

My feelings exactly about the Scarpetta series. How long is the good doctor going to have to suffer? She has to be the most unlucky, unhappiest woman alive.

I have found a great author who is a brilliant series writer. Lee Child's books about former military policeofficer Jack Reacher is a refreshing change. After I read one of his books, I find that anything else I start to read afterwards seems elementary in comparison.

I agree about Scarpetta. Really enjoyed the first couple, then fttttt.

I agree with the above posts about the Kay Scarpetta/Patricia Cornwell series. I guess even Cornwell must have got sick of Kay because she started the Andy Brazil series, but I just couldn't get into those at all.

John Grisham is another one whose earlier books are better. I still enjoy his books but they don't seem as good as his early ones.