For example, can a book about Salem witchcraft written in 1865 still be considered an acceptable source if it provides direction to its primary sources and generally meets the requirements of a scholarly publication?
If a historian from Harvard in 1865 quotes Cotton Mather and offers a reasonable interpretation of the writing, but a historian from Harvard in 1965 uses the same Mather quote and offers a different reasonable alternative, does the 1965 book trump the 1865 book?
In general, a good rule of thumb is that the newer the source is, the more likely it is to reflect the current state of research on the subject. Obviously, you have to read every article or book critically and evaluate their argument, no matter the age (because even now people write just as much bad scholarship as good scholarship). However, it is often important to read older works to understand the development of certain ideas. It is always best to read the original paper or book which expressed an idea or quote that is cited in a later paper. You never know if a scholar might accidentally use it out of context or misspell something by accident (slip of the fingers and all that). For that reason, I have read a lot of works ranging quite far in age.
In my subfield especially (archaeology), we work a lot excavated material. Because we don't always get to look at the excavated material itself, or get to visit the site or see the material being excavated, we have to read the reports of the excavation which are published by the original excavators. This gives us an overview and explanation of the context of the material which we are studying. For an explanation of why context is important, check out this Monday Methods essay from a couple weeks ago.
Some of these excavation reports (or "site reports" as they are sometime called) are from excavations conducted some time ago. Many important excavations were conducted in the 1960s and 1970s, and even earlier in the 1920s and 1930s. For one paper I had to write, I had to consult a 1912 document by George Reisner outlining the discovery of the Samaria Ostraca, as it was the main record outlining the locations and descriptions of the finding of the ostraca. I've had to consult many excavation reports, from a wide swath of time. Of course, at the same time, you have to consult modern works, which all consult the same older works, It's a mountain shaped like an upside-down triangle, of works referring to older works referring to older works. Obviously, you often deal with people reviewing older works and finding their conclusions to be in error, based on more recent scholarship or excavation. That's part of the academic endeavor. We're always trying to reevaluate what has been learned and reinterpret it in the light of new information and ideas. So it's really a mix of old and new ideas constantly being reevaluated. Sometimes, old sources provide a lot of important information, but their conclusions are faulty because new information has since been discovered. But it's still worth reading the old ones, so we can maintain the link of constant correction.
If you want to see what an excavation report looks like, you can download copies from the site at which I work, which are provided for free under the Creative Commons license:
In general, the further a source is from contemporary historiography, the more skeptically I view it. Scholars of the past worked in different ways, with different biases, than we do today. In particular, approaches to race and gender from before the 1970s were very different than today, and it's important to keep that in mind.
In the specific example, both 1965 and 1865 are pretty far from the present. I would tend to hope for something even more recent in student writing that I was grading. If there wasn't anything more recent than 1965 in existence, I would consider that a great opportunity for a new intervention in the field :)