i have threads from years and years ago ~ they have been stored properly, and are still soft and strong. You can feel when thread dries out, then it is weakened and will snap and give you all sorts of fits. But you can sometimes rejuvenate an old spool by sticking it in the freezer overnight.
I have threads that say $.15 on the ends!!! my grandmother lived thru the depression, she never threw anything out! So I have thread that she bought at Woolworth's in probably the 40's and 50's, and because of the way she stored it, I am still working on using it up :)