If modern emission standards are really important then we should enforce standards but not throw away cars. Maybe old cards are still good enough. Maybe old cars can be tuned/upgrades/switched to another fuel etc. Car age is not important at all, and should not be considered.

That is what is done, age is an approximation.

So far, these rules aren't enforced against cars in England, only buses and heavy vehicles in London must comply. It's similar in some large cities in Germany and Sweden.

Example: http://urbanaccessregulations.eu/quick-guide-key-schemes?cit...