“Over the hill” is a subjective term meant to imply one is past one’s prime age. There is no specific age, although research suggests one is cognitively over the hill at 24.
People have been considered over the hill at various ages throughout history. According to Merriam-Webster Dictionary, the term “over the hill” has been in use since 1946. Newspaper clippings from that year can be found referring to outgoing college seniors, students in their early 20s, as over the hill. On the older end of the spectrum, a television movie debuting in 1969 called “The Over-the-Hill Gang” featured elderly, gray-haired men working as Texas Rangers. The term will always be somewhat subjective. As medical science progresses and life expectancies increase, the perception of what is “old” will continue to change.
Generally, one is considered over the hill after passing one’s physical prime. Some people may consider this to be 30 or 40, while others would say 50 or 60. Many athletic or otherwise active seniors would place it even later. In a recent study, however, researchers at Simon Frasier University in Canada found that cognitive performance peaks at age 24. The researchers studied game replays from the video game "StarCraft II," tracking trends across different age groups. They found that players age 24 and older had experienced greater delays than younger players when reacting to the game, and that the deficiency only got worse the older the player was. The study has had a huge presence in newspapers and magazines, which have used it to define 24 as the age one is cognitively over the hill.