What does over the hill mean?

"Over the hill" can mean literally over the top of the hill and down the other side. It can also be an idiom meaning that someone is old. The image is that you are past the "hill" of middle age and on the way down the other side to old age. This can be a serious statement, but is usually a humorous one, where someone tells a friend they are "over the hill" at an early age.