Age alone doesn't qualify something as historic. That is both opinion and city ordinance. Otherwise, a 150 year old wooden shack would out-qualify any 75 year old quality-built, architecturally significant building (edited the dates to put in context with Denver history).
https://www.denvergov.org/content/de...ce_Update.html
To save you some time, here's the criteria.
Landmarks must meet at least three of these ten criteria.
1.Have direct association with a significant historic event or with the historical development of the city, state, or nation;
2.Have direct and substantial association with a recognized person or group of persons who had influence on society;
3.Embody the distinctive visible characteristics of an architectural style or type;
4.Be a significant example of the work of a recognized architect or master builder;
5.Contain elements of design, engineering, materials, craftsmanship, or artistic merit which represent a significant innovation or technical achievement;
6.Due to its prominent location or physical characteristics, represents an established and familiar feature of the neighborhood, community or contemporary city;
7.Promote understanding and appreciation of the urban environment by means of distinctive physical characteristics or rarity;
8.Represents an era of culture or heritage that allows an understanding of how the site was used by past generations;
9.Be a physical attribute of a neighborhood, community, or the city that is a source of pride or cultural understanding;
10.Associated with social movements, institutions, or patterns of growth or change that contributed significantly to the culture of the neighborhood, community, city, state, or nation.