As a lifelong runner it's best to do it by mileage not age. Replace them somewhere between 300 and 600 miles (your weight, leg length, form, surface, etc. all influence this). I will add that over time historically the midsole foam usually degrades so age is a factor too. I'd probably suggest tossing them after 1.5 years or so or at least having a fairly fresh pair to compare them to. The above mostly applies to running but should be relevant for walking too.
Yes exactly, I sometimes push them a bit longer then your upper bound as well to save money but you can really tell the difference once you get a new pair.
My current pair is far overdue! Anyone here have experience with the Gore-Tex 880's, are they good?
Most athletic shoes have a short life like that. The trend tends to wear off. Even if it doesn't, you generally loose grip as the compound ages. Also, the insole and other cushioning stuff tends to break down and loose it's ability to absorb shock, which can lead to joint and back pain.