There is already an age limit in place for existing diesel cabs, it is being reduced from 15 to 12 years for earlier models - additionally, TFL will pay up to £10k to delicense caps not built to the latest standards.
Do you have a source for that? Dropping that age limit from 15 to 12 years would obsolete about 20% of the fleet overnight, since the age spread of the cabs is fairly linear.