I use old thread all the time, including thread on wooden spools. I test it first by holding a length between my hands and giving a strong tug. If it doesn't break, it can be used. Some of the thread that I've used in my art quilts are probably close to 100 years old and they went through my machines just fine, including doing FMQ with them.
That said, I would not use thread that old in a quilt that was going to be used and laundered regularly as I don't do any tests to see how well it holds up to modern laundry detergents, etc.
Rob