What age range is considered "middle-age"?

What age range is considered "over the hill"?

What age range is considered "senior"?

Is "over the hill" and "senior" the same thing?

What are the age-based generation names (e.g., Generation X), and what years do they represent?

I've heard different answers to these questions, so I'm hoping that someone can answer them as accurately as possible.

I had no idea that when I turned 40 years old five years ago I was considered middle-aged. Now I'm curious about the different age groups and when I'll hit the "over-the-hill" mark.