Originally Posted by
RubeRad
If the cables are 3 months old because the bike is 3 months old, then this is 99.99% certainly regular cable stretch that happens with all new bikes (and new cables on old bikes), which is why bike shops (should) always tell you to bring the bike back in a few months for a quick tune-up (and usually do that service for free, for bikes bought from that shop)
I've broken rear shift cables completely in under 3 months / 2500 miles and now pro-actively replace at 2000 miles
Under bar-tape routing with a tight 90 degree guide in the shifter body almost cut my cable life in half as one expects from double the fatigue cycles. Before fatigue only came from wrapping around the shift drum, although now each shift also flexes the cable as it runs over the guide.