Houses have general wear and tear that need to get fixed. Housing designs and styles go out of date. Code gets updated to add new standards such as better insu

Houses have general wear and tear that need to get fixed. Housing designs and styles go out of date. Code gets updated to add new standards such as better insulation, earthquake resistance, plumbing. Roofs need to be replaced, walls need to be repainted, floors need replacement after general wear and tear. Electrical standards change since our demands have changed over generations. And most units don't keep all of this stuff up to date through renovation other than the bare minimum. Like a used car. Since buildings last a long time, a 1920s-1970s building is what is considered an aged building. 1980s is on the edge and 1990s is relatively new. A older building that was overengineered will retain better value and so on.