I wonder at what is the age (and SD?) that a building is decided to be an historical artifact that we're not allowed to restore? The Giza pyramid, the Acropolis

I wonder at what is the age (and SD?) that a building is decided to be an historical artifact that we're not allowed to restore? The Giza pyramid, the Acropolis in Athens, and many more like it are just left to decay. But we constantly maintain buildings like the Eiffel tower and thousands of historical buildings in cities. I think it's a shame that some buildings are relegated to being ruins, rather than being actively maintained.