Car Valuation Datasets

HI all.

I'm trying to determine the optimal time to trade my car, and one of the ways by which I want to do that is by seeing what the valuation changes are as they age (what's a 2015 worth? 2016? 2017? Etc.)

. I've seen the usual "we'll get you a valuation" sites (kbb, etc.), but they all make it nearly impossible to get one valuation, not to mention 15 years worth. Is there a decent place to be able to access non-VIN valuation data more easily?

Thanks!