A. A good quality thread that is produced today will last much longer than thread which was produced 15 or 20 years ago. Even the best quality cotton thread of a generation ago did not have the advanced processing techniques available to us today and it would probably be best to avoid using them. However, a top quality cotton thread manufactured today, like MasterPiece and King Tut, will probably be fine to use 40 or 50 years from now. The difference is due to a higher quality cotton and advancements in spinning, dyeing, and twisting technology. Since Cotton is a natural fiber, it will degrade over time. A good test to check whether or not the cotton threads you have are OK to use is to hold about a one foot section in both hands and pull apart. If the thread snaps (you should feel a nice, crisp break), then it is OK to use. If the thread just separates and pulls apart easily, I wouldn't use it.
As for polyester thread, the color may fade over the years with exposure to sunlight but there is no evidence that the thread deteriorates so it will last a lot longer than we will.