I get asked about old tubes enough in the shop that it's worth posting about it here. NOS tubes are a gamble, as are any tubes. Just because they were made 40 years ago doesn't mean they are any better than modern tubes as a rule of thumb. Nor does it make them worth a mint.
Tubes have always been manufactured to a set of design specs. This info is found in the data sheets. For example, here is a sheet for an old Tung Sol 6V6GT:
http://www.tungsol.com/tungsol/specs/6v6gt-tung-sol.pdf
You can find everything you need to know from operating voltages to warm up time (note it's only 11 seconds, not the poor info you see regurgitated on forums like 5-30 minutes). Occasionally, if there was a specific set of demands for a particular design they will show up as well. Notice that there is typically no published spec for lifespan. That's because real world applications run tubes under different operation conditions. Some circuits are far kinder to tubes while others pummel them within an inch of their life (a Deluxe Reverb is an example for 6V6's, as would be a Class A SE amp).
For an example of a special spec, check out the 7320 (long-life EL84 variant) data sheet:
https://frank.pocnet.net/sheets/128/e/E84L.pdf
Note the section on the top right about reliability (measured in how many of 1000 tubes fail over 1000 hours) and long-life (10,000 hours average life expected over 100 tubes). Typically "mil-spec" tubes were outliers on a bell curve that exceeded the normal production specs and met a standard set by the military. They were produced in the same batches as all the other tubes, they were just the top performers of the batch.
As for NOS tubes today, this market has now been combed over for 30 years. A lot of the best remaining stock was sold first at this point and a lot of what I see being sold recently tends to be leftovers. My observation based on the thousands of tubes I seen through the shop is that if an old tube is still functioning today, it was probably somewhere in the center of the bell curve of reliability. Do they sound better? Sometimes. Probably not quite 50% of the time even. Are they worth a mint that folks pay on the open market? Well things are only worth what the market will bear according to economics.