How old is "old"?

  My Sunday school classmates have kids older than i am.  But somehow
i seem to them more mature than their own kids.
  Well, they remember their kids as, well, kids.  To a certain extent, that's how we always think of our kids.
  My mom said i'd always be her baby.  Not the most enjoyable thought....

Age thrrough One Writer's Eyes

   We can see this type of idea in the writings of Agatha Christie, 1890-1976.
  It's been awhile since i came across this reference, but in one of her earliest works, she uses the word "elderly" to describe a man under 50 years old.  This would be a work written when she was in her 20s
  In 1936, she describes character Louise Leidner as "no longer young, between 30 and 40," but twenty years later, Mrs. Upjohn, in her late 30s, is "a young woman."
  Tommy and Tuppence Beresford were characters she didn't often employ. In the 1920s-written novels, they are a breezy, adventurous young couple, certain that youth is superior to old age.  In N or M?, they are in their 40s, chafing at being unable to help in the war effort.  They see themselves being just as good, better even, than in their Young Adventurer days.
  In By the Pricking of My Thumbs, they actually are elderly.  It is one of the last novels Christie wrote.  She opens by with them breakfast, and comments that, to an onlooker, they would appear to be an ordinary, boring old couple.  However, they themselves knew that young people don't know anything!