Mexivoof, this is not always the case since Biltmore House has been built about around 1900 and it is still copyright protected since it is private property. Sometimes architects protect their 'work' by copyright (e.g. Hundertwasser-buildings are copyrighted) and the light-work of the Eiffel-Tower in Paris is copyright protected as well. Age is not really a criteria.
I would just like to know if I could have a legal issue with the Space Needle since so far I didn't find any source for that.