They built it 10 years ago.

[diamondcutter]

Tom: How old is this house?
Jack: They built it 10 years ago.
(Made by me.)
I used the second sentence to mean “The house was completed 10 years ago.” Did I use it correctly?

 

[Tdol]

Yes

 

[emsr2d2]

It's OK but it's not a very natural answer to "How old is ...?" The answer to that question usually involves the word "old" (if it's a full answer), and just a number if it's a shorter answer.

Q: How old is the house?
A: [It's] ten years old.

Q: How old's your son?
A: Twelve.

If the question was "When did they build the house?", the natural answer would be "[They built it] ten years ago".
If the question was "When was the house built?", the natural answer would be "[It was built] ten years ago".

 

[Holmes]

Tom: How old is this house?
Jack: They built it 10 years ago.
(Made by me.)
I used the second sentence to mean “The house was completed 10 years ago.” Did I use it correctly?

Yes. The sentence "They built it 10 years ago" implies that the house is 10 years old, stating that information in a different way.