over the hill, to be

be over the hill

To be past one's youth. Although often associated with age 40, this phrase is not tied to a specific age. Now that I'm over the hill, I'm sore for days after I work out. I'm not looking forward to my next birthday because I'll officially be over the hill.
See also: hill, over
Farlex Dictionary of Idioms.

over the hill, to be

To be past one’s prime. The analogy to a climber who has reached the top and is now descending has been transferred to the decline of aging since the mid-twentieth century, principally in America. Mary Roberts Rinehart used it in The Pool (1952): “The flawless skin goes, the lovely eyes fade, and she knows she is over the hill.” However, in U.S. military slang during World War II, going over the hill meant going AWOL (absent without leave).
See also: over
The Dictionary of Clichés by Christine Ammer
See also:
  • be over the hill
  • at a tender age
  • tender
  • tender age
  • tender age of
  • the big (number)-o
  • the big something-o
  • Catholic twins
  • of age
  • at the tender age of