Until the early 1990's the printing was done with a heat set process. There was no process for a washout ink. Report over the years have said only with repeated washing will they lighten but won't totally disappear. I assume it was the assumption that the quilter would cover the marks in the process of appliqué. Am afraid you are stuck with the marks.
It has nothing to do with age but the printing process. The wash out products became available in the early 1990's with the preprinted wholecloth designs. It was a few years after that that the companies printing embroidery blocks caught up with the process.