I don't think yarn gets 'old', unless its wool that has had moths. Some natural fibers will break, especially when thin. What I like to do with natural stuff is a 'felted join' or 'spit join.' Cut out the part that has the broken ply. Fray the two 'good' ends for about an inch. Then lay one on top of the other and moisten. Rub the ends between your palms vigorously. They should felt together enough to knit with.
If it doesn't work, you can tie a loose knot, knit that part, take out the knot and weave in the ends. Or just start the new end as if you were starting a new ball.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry