I don't know what your models are doing, but if you look at e.g. a ten year old Ford Focus, the original price was somewhere around $17,000. With only one mile on it the bluebook is now around $3800, with 120,000 miles it's around $1800. Which implies that being ten years old accounts for something north of $13,000 of deprecation and adding 120,000 miles accounts for only $2000 more.

That may be different for cars bought after they've already suffered most of the age-related depreciation they ever will, but in that case there is only modest depreciation in either event because the car can't lose much value to depreciation if it was already not worth very much when you bought it.

A ten year old Ford Focus with one mile on the odometer isn’t realistic. Download real data from cars.com by scraping the HTML and build a linear regression.