Although the heading specifies tennis shoess, I'm writing this with all types of sports shoes in mind : tennis shoes, walking shoes, sneakers, aerobics shoes, basketball shoes, etc . The principle is usually the same. For brevity, lets just refer to them for the most part as trainers.
In my office, I most typically will have people let me know that their sneakers are many years old BUT ( insert excuse here... ). Some of my favorites, "it fits my feet completely" ( meaning it's broken in ), the heel is worn out just how I like it ( meaning it has got a hole inside the shoe ), new shoes just bother me ( meaning that I have used these for so long, the support is useless at this point ), and my most favorite : "they continue to look new".
The action and function of a running shoe has little or nothing to do with the general look of the upper part of the shoe or even the sole. It is the midsole, the part of the sole that is mostly invisible from the outside, that does teh most action and actually restricts the helpful lifetime of sneakers. With each walk, the midsole is squeezed and, over time, loses the majority of its support. I guesstimate the useful lifetime of a running shoe for road runners to be only 400 miles. This distance could be a touch shorter for larger runners and a touch longer for lighter ones.
Yes, they still look new. When all you do is run on the treadmill, use the elliptical, and some stationary bike, and them sneakers have never seen the light, naturally they look new. What's problematic is the part you don't see.
Dedicate 2 shoes ( or more ) totally for exercise to get the most out of its helpful life. You can record your usage in your running journal, or even in pen on the side of the shoe. Mark the date of the tennis shoess on the tongue or the outsole of the shoe so you really know how old it is. Whenever you are involved in serious exercise, minor wounds are unavoidable. You must do everything in your power to minimize them.