How can I tell if a paper is too old to be a reference for my research?
The age of the paper per se should have no relevance. However, the researcher had better be certain about whether the conclusions of the old paper have been superseded or even invalidated by more recent work. This sort of problem is unlikely in, say Mathematics, but other fields can be different. There are Philosophy papers that, to my knowledge, have invalidated old work. The old work said "X is impossible". The new work said "X is indeed possible and here is an important example." This wasn't mathematical philosophy (logic) of course, but observations about learning (epistemology). 

The fact that a paper hasn't been cited in 50 years is of little importance, especially in a narrow subfield  of some larger field. The fact that it is no longer relevant is of vast importance. However, there are some studies intentionally done to compare the old with the new and here, citing both is the essence, of course. 

One reason for not finding old papers is just the difficulty and expense of properly indexing them in databases. For "popular" things someone is likely to want to go to the trouble, but for arcane things, not so much as there seems to be little payback for the effort. 

And yes, you normally don't need to go back before the Principia, but, you know, Euclid was an interesting guy. 



Note that another question here asks about finding more recent papers than the one you are reading.