Is 57,000 miles on a 2007 Mustang still good?
Most people average between 12,000 to 15,000 miles on their car per year. Theoretically this car could be 5 years old if it was originally purchased new early in the model year, late 2006.
I would question why a 5 year old car had only 8000 miles. That is only 1600 miles per year.
The other car with 57,000 miles has average miles. It was driven about 11,400 mils per year. This is well within what would be considered average miles for a 2007.
30 years ago cars had issues around 70,000 miles. Modern cars this is doubled. Many times they will be completely reliable past 140,000 miles and can sometimes go past 200,000 miles without any serious issues. Of course this all depends on how well the vehicle is maintained and how hard it is driven.