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I read somewhere recently that there were better times to sell your car based on it's age, I seem to recall they suggested when it was 8 years old was the ideal time. Is there a 'best' age to sell cars at?
No not really, mileage can be equally important.
Most people accept vehicles up to around 100K still have some life left, but once they reach that 150K mark and above, prices and interest can change.
Working on your 8-year theory and the national average of 14K per year, I guess it is a good time to consider selling to help create buyer interest.
Buyers are also looking more for safety and economy features, so the older a car gets the less likely it is to appeal in this regard.