Tube Measurements?

Once a tube is measured, do they stay the same over time? that is, if I buy an NOS tube that measures xyw today, will it still be the same 5-6 years down the road when I need to use it. I wonder about this in cases I want to buy some NOS that are running out and what to buy now and use later.
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I agree with Elizabeth. I've had occasion to test a considerable number of 50, 60, and 70 year old NOS tubes, on a Hickok dynamic mutual conductance tester, and they have tested as I would expect a new tube to test.

Also, the prices some audiophiles are paying for certain tubes of that period would seem to provide added confirmation.

-- Al
Ditto the above.I have several 30-40 year old tubes that test to new tolerances.
That is good to hear. I've been building a "stash" of NOS with the next 20 years in mind, and wanted to be sure that how they tested when I bought them would be essentially how they will "test" when I using some years down the road. Thanks for you input.