I, too, inherited a good bit of old thread from my MIL and have slowly used it as long as it seems to be strong enough for the project. Snap it good to make sure it doesn't break--if it doesn't, it is probably OK to use, at least for hand sewing on bindings and the like. Sometimes even OK for my machine, a Singer. I'm sewing as fast as I can to not leave much old thread left when I'm gone!
Empty spools are great for crafts for kids. Remember making animals with them? Spool is the body, with cardboard front and back legs, yarn or string tails, etc. My First Graders at school have loved making horses that way! Lots of good fine-motor workouts for those little hands, and better than video-game-thumbs!