The emissions regulations of fifteen years ago are much more relaxed than today's, and of course old cabs are measured to the old standard- grandfathered if you will.
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.
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.