It depends on your financial status and the condition of your car. Lower mileage has more value on older car; but year model also has more value on younger car. In general i think big companies sets the line.
Past the two year mark, that value of a car drops fast. Past 150,000 km the value also drops fast.
80,000 km or 2 year is my border. No maintenance yet, still some warranty left, good option for buyer. All gives me better price. I usually find I have to pay about 20-25% of the new value in; often even trade in at better price than original price paid.
BUT - if your car is on lease (Not paid off) better keep it and pay off, then enjoy a few months without those payments before you consider trade in.