Do you know of anyone who has been well educated,intelligent,smart,professional,but somehow began to lose it?

I have known many friends,acquaintances,relatives who were smart and popular at school, got good paid jobs, were very intelligent until they reached middle age where they lost the plot and became mediocre. Through some sort of unexplained circumstances and events they just lost their level of intelligence, became apathetic,got retrenched or laid off and were below normal people in the society.
It is as if they could no longer adapt,compensate or improvise to the level of technology, resources,
information and au courant.

I am not referring to any mental illness or some sort of psychological disorder but life experiences in general have not been good to them.