A 2015 car is now three years old. When does the asking price of a car reflect the three year price rather than the two year value?

In 2017 the average price for a 2014 Mercedes-Benz E-Class was currently $35,000 and a 2015 was $45,000 while a 2013 was $24,000, a $10,000 price differentially year. It is now 2018 making all of these cars a year older. Therefore, when does the price differential of $10,000 per year begin to be applied reflecting the fact that the cars are a year older which impacts warranties, etc.

Aye. It doesn't work like that. Cars do not depreciate at a set value amount each year. Depreciation happens every second of every day and is completely variable by each individual vehicle's condition, mileage, maintenance, etc. You are very much overthinking the depreciation. If you're interested in a car, make an offer on it. The seller can accept or refuse any offer. A car, ANY CAR, is only worth whatever the market says it is, which varies for each transaction made. There's too many variables to set even a remote time frame on depreciation.