[garciansmith]

In the United States something simply being old in no way makes it historic in any legal sense (e.g., contributing to a local historic district): integrity[1] matters if you are trying to legally deem a resource historic and worthy of some sort of preservation effort. Generally speaking buildings, structures, or objects need to be at least 50 years old, integrity aside (but there are exceptions if they are particularly noteworthy).

I tend to disagree with the need to tear stuff down just because it's old: it's so terribly wasteful. We need to get better at adapting, reusing, and adding on to older buildings. Granted, when developers just want to use the cheapest materials possible and build something that will start to have serious problems in 20 years, it's a problem, to say nothing of the loss of serious knowledge in various skilled trades.

[1] In most cases the National Register of Historic Places aspects of integrity are used for evaluation: integrity of location, design, setting, materials, workmanship, feeling, and association.