Yeah, don't throw something out because its old, but definitely try it first. I seem to recall industry recommendations of shelf life of 5 to 20 or so years for varnish, but I'm going from a slightly faulty memory. But it can last longer, but I wouldn't bet stale bread on hundred year old varnish being any good.
As far as what goes bad, when it comes to petroleum, gasoline goes bad but I think mineral spirits is pretty stable. I would guess the resins are relatively stable too. On the other hand, linseed oil is a vegetable product. It might have a long shelf life, but I don't see it lasting forever. How long do canned vegetables last? The other question, do petroleum products affect its stability? I don't know.
Tim
