Originally Posted by
FBinNY
+1, if the ozone and UV have them to where they've dulled considerably they're nearing the end of their life. This isn't an issue with brake cables who's plastic covers are only cosmetic, but is a problem for derailleur housing where the outer plastic cover is structural, buttressing the long spiral steel so it cannot bow out and collapse.
Of course the OP has an old 10 speed, which does not have specific brake and gear housing. Regardless, I agree with everyone who says to just replace it.