When you are young, and you realise something true and important, this is evidence of how clever you are, even if what it is that you have just realised was really rather obvious. (And everything is obvious, once you’ve understood it. That’s what understanding is.)
When you are old, however, and you realise something true and important, this is evidence of how stupid you are for not having understood it about forty or fifty years sooner than you did. (Because everything is obvious, blah blah.)
This has happened to me twice in the last fortnight. I will not complicate this posting by confessing what these two very different but very obvious things were, but trust me, they were very obvious indeed.