I’m not a consistent cross-stitcher by any means, so I assume some of the thread might be from when I first picked up the craft in junior high or high school. Some will date to each time I picked up the craft again in the intervening 25 years or so since I left home.
In general I don’t think it matters to me how old my thread is, but I know some people are curious about how old their thread might be if, say, they purchased it at a garage sale. You might also be stitching a vintage project or finishing a project someone else started and want to try to use thread that would be contemporary to the design.
Lord Libidan has a great post with a lot of pictures all about how to estimate the age of DMC floss based on the label. For example, the oldest DMC threads were sold as spools, and the oldest skeins of floss, which were produced beginning in 1910, were 7 yards long instead of the 8.6 yards used today. (It was an update to the metric system, where 8.6 yards is equivalent to 8 meters).
The post talks about and shows in photos where applicable the different ways the wrap labels have looked and what information is on them so if you have these wrappers you can estimate how old your floss is. Check out the post at Lord Libidan for all the details.
You might also wonder if it matters if you use old floss or combine old and new floss in a pattern. From what I’ve read there’s not necessarily a degradation in quality from using old floss, but it’s a good idea not to mix really old floss and new floss in the same project because they will look quite different because of the differing dye methods or treatments that might have been done to the floss in different production eras. Also of course dye lots will be different so even if you have the same color from different eras it’s unlikely to match perfectly.
[Photo: form tyrsa on Reddit via Lord Libidan]