Gretchen and Henry were looking to buy a house together, and finally found one with a beautifully landscaped yard, plenty of bedrooms, and a beautiful tree out front. They asked the owner how old the house was, and this was his reply:

"I planted that tree when the house was built. It was 7 feet tall at the time. It has grown 3 feet each year, and at the end of this past full year, it was four times its original height."

"But how old is the house?" asked Henry.

Gretchen said, "Don't be so dense. He just told us."

So, how old is the house?