I have a Ph.D. in anthropology and I have worked as a professional archaeologist in academic, government, and cultural resource management (CRM) roles in the United States and Mexico. I want to first address your statement about the excavation of burials of great age being considered acceptable. The first thing you need to know about excavating human remains in the U.S. is that it is never done without a very specific reason or without approval from the government, Tribes, and/or descendant communities. From my experience, excavating burials, regardless of their age, is often very contentious and in the government and CRM world, disturbing human remains is generally avoided at all costs. Here in the States, this is in part due to the Native American Graves Protection and Repatriation Act (NAGPRA) and a number of state laws which protect remains and rightfully give descendant communities a say in the treatment of any remains discovered during archaeological research or other activities (e.g., construction, floods, erosion). I have worked extensively in the western U.S. where the Tribes are very active in the archaeological community and I have rarely seen a case where they have signed off any non-essential disturbance of human remains, especially any form of destructive analysis (e.g., radiocarbon dating, isotope studies). Long story short, excavating burials of great antiquity in the U.S., which are undoubtedly associated with Native Americans, is not something that is common or without controversy. However, you may find that other cultures or governments have different opinions about the treatment of the dead in archaeological contexts but that is not something I can speak to.

In terms of an age limit, most states in the U.S. follow that any item, structure, or building greater than 50 years old is considered a cultural resource. I have never seen that rule applied to human remains because archaeologist rarely encounter situations where they are dealing with or researching mid/late 20th century burials except in very specific cases. Ultimately, when it comes to doing academic research on human remains, it is not the age of the burial that is important (in most cases) but what that research can contribute to our understanding of the human condition. Therefore, before any burials can be excavated, the researcher(s) must develop a specific research question and a detailed plan on how they plan to answer it by conducting that excavation. After that, the plan will be critically evaluated by government agencies, ethics boards, and/or descendent communities before a shovel ever touches the dirt. In sum, conducting research on human remains is heavily regulated and it is not the age of the burial that is important, it is the scientific value of excavating that burial.