it is too old if when held in two hands and given a tug it breaks.
I have been using threads my mom bought from some going out of business place 20 years ago= the thread is good-strong thread-she has always taken care of it.
i have bought thread that within a year was no longer usable thread.
there are textiles in museums with threads 100's of years old and still holding up.
there are clothes in my closet with weak, old broken threads with in months.
the only way to know is to test it for strength-color fastness (is it faded out & darker as you use it off the spool?)