That brilliant idea that you had yesterday afternoon? You may remember it now as being very insightful. Unfortunately, that may be all you recall.
I have a very good memory and yet the number of thoughts that can sort of drift away is formidable. That's why a Moleskine or a notecard is usually within reach. It may be a sign of geezerdom to be scribbling away in a coffeeshop but who careth? Those scraps of paper get the job done.
Comparing my memory now to what it was when I was 20 is unfair. There were fewer ideas and responsibilities back in those glorious days. No wonder a sign of youth is the belief that they know it all.