I think. I think that as many grow older they lose friends and have less social interaction.  That loss of interaction means less usage of thought and vocabulary.  Just as doing less movement or work results in losing strength, agility, balance so does less interaction result in loss of vocabulary and cognitive thinking.

It appears to me that those who live longest and retain mental acuity are also those who are active physically, emotionally and socially.  Looking at the nightly news it's apparent that isn't always the case but locally it seems to be that way.