2.280 indicating something's age

If you want to say what the age of a thing is, you can use

* 'be' followed by a number followed by 'years old'

• It's at least a thousand million years old.
• The house was about thirty years old.

Note that you can also use this pattern after a noun.

• ...rocks 200 million years old.

* a compound adjective indicating the century when something existed or was made, which consists of an ordinal number and 'century'

• ...eighteenth-century chairs.
• ...a sixth-century church.
• ...life in fifth-century Athens.