The year a property was built can be found in the deed history of the house or building. Usually these documents are in the hands of the property owner, but if they have been lost or destroyed, a copy of the deed is usually retained by county recorder.
If a deed to the property in question cannot be found, there are several other methods of narrowing down the date in which it was built. Sometimes the concrete foundation has the date it was erected stamped on it, which should coincide with the date the property was built. The local historical society and the tax recorder's office may also have information on the date the property was built.