> which can pose a significant (and growing, as standards improve) obstacle for older cars.

At least for my state, the emissions test a car has to pass is whatever it was supposed to have passed when it was fresh off the assembly line. So older cars do not have to pass stricter newer standards that newer cars have to pass.

Now, granted, wear and tear will eventually result in an older car not passing its original standard, but at least the standard it has to pass is fixed, rather than a moving target.