[yangyang]

It would also be nice to have the option of using age instead of mileage, or possibly miles / year.

Sometimes an old car that's hardly ever driven, with a low mileage, isn't as good a bet as a newer car with lots of miles that's been well maintained.

[msellout]

That's true, but age is so strongly correlated with mileage that it's an insignificant factor in sell-price once you control for mileage (Based on my own research using proprietary data).