In 2014, science came to the conclusion that the elderly have trouble remembering because they have filled their heads with so much information from their long lives.

Well, Sherlock Holmes Knew This by 1887.

In A Study in Scarlet (first published around Christmas 1887) Sherlock Holmes says,
“I consider that a man's brain originally is like a little empty attic, and you have to stock it with such furniture as you choose. A fool takes in all the lumber of every sort that he comes across, so that the knowledge which might be useful to him gets crowded out, or at best is jumbled up with a lot of other things, so that he has a difficulty in laying his hands upon it. Now the skillful workman is very careful indeed as to what he takes into his brain-attic. He will have nothing but the tools which may help him in doing his work, but of these he has a large assortment, and all in the most perfect order. It is a mistake to think that that little room has elastic walls and can distend to any extent. Depend upon it there comes a time when for every addition of knowledge you forget something that you knew before. It is of the highest importance, therefore, not to have useless facts elbowing out the useful ones.”
The above remark must have taken place by 1882, probably in 1881
Watson met Holmes in A Study in Scarlet and then later writes, "When I glance over my notes and records of the Sherlock Holmes cases between the years '82 and '90..." (from the beginning of "The Five Orange Pips").

Then later in "The Five Orange Pips" (which Watson dates from September 1887; the following is about two-thirds of the way through the story):
"Sherlock Holmes closed his eyes and placed his elbows upon the arms of his chair, with his finger-tips together. 'The ideal reasoner,' he remarked, 'would, when he had once been shown a single fact in all its bearings, deduce from it not only all the chain of events which led up to it but also all the results which would follow from it. As Cuvier could correctly describe a whole animal by the contemplation of a single bone, so the observer who has thoroughly understood one link in a series of incidents should be able to accurately state all the other ones, both before and after. We have not yet grasped the results which the reason alone can attain to. Problems may be solved in the study which have baffled all those who have sought a solution by the aid of their senses. To carry the art, however, to its highest pitch, it is necessary that the reasoner should be able to utilise all the facts which have come to his knowledge; and this in itself implies, as you will readily see, a possession of all knowledge, which, even in these days of free education and encyclopaedias, is a somewhat rare accomplishment. It is not so impossible, however, that a man should possess all knowledge which is likely to be useful to him in his work, and this I have endeavoured in my case to do.'"

At this point Holmes refers back to the discussion he had with Watson in A Study in Scarlet about Watson's questioning Holmes about what he didn't know and his answer.

“'I say now, as I said then, that a man should keep his little brain-attic stocked with all the furniture that he is likely to use, and the rest he can put away in the lumber-room of his library, where he can get it if he wants it.'"

Sherlock reminds us of this in 2010 in episode #3 "The Great Game":
Sherlock points to his head as he tells John, "This is my hard drive, and it only makes sense to put things in there that are useful — really useful. Ordinary people fill their heads with all kinds of rubbish, and that makes it hard to get at the stuff that matters.

Sherlock Holmes did not have to wait until he was elderly to realize this fact because he learned so many things when he was young: even if he only learned things related to detection, he needed to learn every detail of that and every detail of anything, well enough all the criminal activity that people have engaged in, is a lot of information.
(Editor's Note: Of all the things Sherlock Holmes ever said, this was the one we felt was least likely to be true; indicating again just how much Sherlock Holmes stands up to reality; sometimes more than a modern person with a reasonable IQ and 18 years of schooling does.)

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