1. Wrong: We wanted to purchase the seven year old car because it was dirt cheap.
Better: We wanted to purchase the seven-year-old car because it was dirt-cheap.
Explanation: Each of the hyphenated words is a multi-word adjective.
2. Wrong: The top of the line model costs more than the new to me junk heap he tried to sell me.
Better: The top-of-the-line model costs more than the new-to-me junk heap he tried to sell.
Explanation: Both multi-word adjectives need hyphens.
3. Wrong: We want to air condition the larger than life English Wing in the Main Building.
Better: We want to air condition the larger-than-life English Wing in the Main Building.
Explanation: Larger-than-life all works together as one adjective, which is why it needs hyphens.