Use the adjective over-the-hill when you need a blunt way to describe someone as really old. You might feel over-the-hill if you can't keep up with the little boy you're babysitting.
When someone is over-the-hill, he's too old to be of much use, like a former football player who's past his prime. It's more common for a person to describe himself as over-the-hill than to talk about someone else this way: "I just turned sixty — I'm officially over-the-hill." It's also commonly used in a joking way. This phrase has been around since about 1950.