There are a lot of issues with his calculations.

For people driving 12,000 miles a year their mpg will be higher, more highway miles.

The 10mpg difference in new car vs old for similarly sized cars is over 20 years. The 2001 impala I used to have got 25 mpg.

People that buy new cars typically have cars less than 10 years old that they are replacing. People typically don't go from a clapped out 20 year old car to a brand new one. The "old" car most people are trading in is getting 30-35 mpg.

I'd put the number at 5-7 years for a car that's less than 5 years old.