When do we become dumb again?

We are brought into this world with a blank slate which we fill up with learning’s from school, family, and life in general.  We mature, have children, they grow up and have children of their own, you retire and then you become dumb again.  After 21 years in the business world with middle manager experience and graduate level education, it suddenly seems that at least to my grown children, I no longer have any business sense.   This fact was reaffirmed to me on my recent birthday when my grown sons were discussing their very busy business life and when I approached thinking I may have something to contribute to the conversation, they suddenly shut-up.  Now, this was not the first time this had occurred.  However, now I am certain of it; I am dumb again.

Somewhere in the last few years, I somehow missed the draining of knowledge from my brain.  Perhaps, it was happening while I was sleeping as I knew it was harder this time in school as I was busy studying for a new second career.   I knew that my two-cents on child rearing was not seen as valuable according to all the new theories of child birth and rearing even though I had two successes in my sons.  However, I had worked in business since I was a teenager and advanced in my profession as I received my education.  This is really puzzling. 

What is the magical age that you become dumb again?  Is it when you officially retire?  Does it creep up on you or does it happen over night?  Maybe I would know the answer to these questions and the universal truths, if I wasn’t so dumb.