Depending on what you're looking for in a car and how much you know about evaluating a used car, 8-10 year old cars can be either a fantastic bargain or a horrible, horrible money pit.
If you've been wrenching for a while and know how to look a car over and know where to go to learn about the weak points for a particular year/make/model, and if you can be patient, there are some definite bargains to be had. With proper maintenance and good treatment, most cars should easily hit 200,000 miles or more so a 10-year-old car with 100,000 miles on it isn't always a problem child.
However, it's also very easy to dress a car up and make it look much better than it really is. If you don't know how to evaluate a car, it's very easy to end up with one that has not been treated well and that has impending issues. That might be OK for something that you're planning on using as a weekend semi-project car, but it's a disaster if you're looking for reliable daily transportation.
The average for a commuter car seems to be around 12,500/year based on what I see when I scan the used car ads. Sportscars tend to be used less, but they also tend to be driven harder and, especially if they've been through more than one owner, have a spotty maintenance/modification history.
Basically, once you're in the 7-8 or more years old for a car, you need to be pretty sure of your ability to correctly evaluate the car's condition and your ability to understand the potential pitfalls that apply to that specific year/make/model. There are bargains out there, but there are a lot more cars with iffy histories that are money pits waiting to happen.
ZV