Anyone who's ever climbed up a hill and gone down the other side knows several things: A.) climbing up is a lot harder than going down the other side. B.) the view from the top is better than the view from the bottom. C.) after reaching the peak, you have passed the highest point, and from then on you must go down.
Some people look at life in the same way. We struggle through youth, and climb in our career with difficulty, but when we reach the peak of adulthood, it is all downhill after that. Physically, we begin to slowly deteriorate, and career-wise, we may get pushed aside as younger people take our place. The only question is: When do we begin "to go over the hill?" Some people say it's at 30 years old, others at 40 or 50, but more positive folks say it is even older than that.