Over the hill is a meaning that you're getting old and your body is tired and worn out and you're unable to do things you used to be able to do when you were younger.
Some people are considered over the hill once they reach 40 or 50 years old and some aren't considered over the hill until they're in their 70's or even older.
It all depends on the person and how their body feels if they think they're over the hill.