If you can fix what's broken, it's sensible to buy a 2-3 year old car and get a decade out of it, then repeat.
Getting rid of a fairly new car (4-6 years old) makes no sense to me. It's always cheaper to drive the car until there's something majorly wrong with it, and it's not worth fixing.
Right now at this moment, I'd still buy new and drive it for 10+ years since the deals on new cars are excellent, while used cars are commanding outrageous prices. Example: A 2008 Honda Fit Sport with 30k miles was selling for $15,500. My fiancee bought a then-new 2009 Fit Sport for $15,900 before taxes. She's put 20k miles on it. The 08 would now have 50k miles, while her 09 has 20k miles. It made better sense to buy new, in that specific case.