If your Teflon pan has scratches in it, its time to replace! Though Teflon pans are higher maintenance (Never use metal utentsils to prevent scratching/breaking the coating), they are a big help in cooking delicate foods like eggs.
We do the bulk of our cooking using our stainless steel pans and only reserve the Teflon pans for eggs and fish (where we don't want to use a lot of oil to prevent too much sticking).
Supposedly, you can resurface your Teflon pan (http://www.ehow.com/how_2093149_resurface-teflon-pan.html). Optimally, you wouldn't replace that Teflon pan with another one. Instead, opt for something cast iron (as long as it is well-seasoned -- not flavored but a process of repeatedly oiling/heating it, it is miraculously non-stick).
Age alone won't indicate that a non-stick pan needs to be replaced but there are 2 key things that will warrant replacement. The first is any scratches in the surface. The second is overheating. If your pan has ever been hot enough to scorch butter then the Teflon coating has already begun breaking down. DuPont doesn't specifically say that you need to replace the pan but does warn that: "the quality of the coating may begin to deteriorate - it may discolor or lose its non-stick quality. This can begin to occur at temperatures above 500F (260C). If heated to an extremely high temperature, the coating may begin to decompose and give off fumes. DuPont non-stick coatings will not begin to significantly decompose until temperatures exceed about 660F (349C)".
As far as ditching them all together? Teflon does emit gasses that are strong enough to kill birds. There was a good reason for keeping a Canary in the coal mine, and getting out asap if it croaked. A study by Environmental Working Group, in collaboration with Commonweal in 2005 found chemicals in the umbilical cord blood of US-born infants including the Teflon chemical PFOA and researchers at John Hopkins found Teflon in umbilical cord blood in 99% of 300 newborns tested. From: http://eartheasy.com/blog/2009/01/healthy-cookware/
That said, I still use them occasionally for Omelets but I always keep the heat below medium.
I suggest you replace ALL your Teflon pans immediately with iron skillets etc. Iron is really good for you, Teflon is REALLY bad for you and the environment. The moment Teflon has one scratch in it it begins to seep toxins into your food.
DuPont, is an unreliable source for information since they invented the toxic stuff. Can't recommend enough that you should replace them all and never using Teflon again.
Personally, I am not a big fan of any type of cooking product that releases foreign substances into the food product you are cooking. Manufacturer's and the FDA recommend replacement when visible cracks or flaws appear in the cooking surface. However, my concern with this product is that residual cracks or imperfections, not visible to the naked eye, can also cause transmission of fluorotelomers, the substance Teflon is derived from to be released into the food. I would stick with good old fashioned cast iron, which after repeated use develops its own non-stick surface and can provide beneficial nutrients to be released into food and not toxins.
I would suggest not using Teflon pans at all. Not only does Teflon emit toxic (odorless & colorless) gases when over heated, but will also flake off into your food (visibly and microscopically) and be absorbed into your bloodstream, and has been though linked to neurological disorders. If there's any doubt, throw it out! Stainless steel or cast iron both only require seasoning to provide little to no sticking, as they can cook foods at much lower heats/flames then aluminum or pot metal coated with non stick surfaces.