When does something become unwearable?

I've got about 2, 3, maybe 4 pairs of jeans that have holes in them mainly in the knee area, I have had these oh about 13, 14 15 years, I still wear them occasionally, Now as long as I don't get a hole in the crotch area where it's beyond repair I'll keep them or I'll make cut-offs out of them . Now when they completely start to fall apart I'll use them for cleaning rags and such.