I'm a bit rusty on my preservation stuff, but I believe in order to get national landmark designation on a national level, your property must be at least 50 years old.

It also needs to meet a bunch of different criteria, in the case an atrium hotel, it would have to be the a really good example of a type, perhaps be the first one in existence and have used a special building material, engineering method or be a fabulous specimen of this type. There are oodles of factors.

In the case of other lesser status landmark designations including ones that are from the National Trust (not the US gov't NPS designation) and involving county, city, state landmark designations, the property/building/site may be less than 50 years old and have different criteria for landmark status.

Please see:

National Park Service landmark designation questions and answers

lala