Maybe for tourist travel it's a cool way to spot old historic buildings you'd otherwise not find, but for real estate? I would expect that whatever real estate catalogue you browse will also list the build year. As another commenter said, that data is in public databases (called the "kadaster"), I would assume every real estate agent is aware and uses the kadaster-data :)

Which doesn't detract from the fact that this is a beautiful visualisation. And also I doubt the general public can query those data in the bulk amounts required for this map :)