over the hill

Over the hill is a common idiom for someone who has reached over a certain age and therefore is considered to be half-past their life span. If you think of your life as being a hill, then once you get to the top it would be the halfway point, and going down hill after that is when you reach past this age. Traditionally, over the hill is being around age 40, but some consider age 50 to be over the hill. The phrase is also used when you can no longer do something well because you are past your prime.