As long as they are road worthy what has age got to do with it?
For private vehicles not much.
However its easy to see why a government might be concerned about older vehicles being less safe (older design, no airbags etc) more polluting and probably more noisy.
There is also the possibility they may for political (or bribery) reasons actually be trying to favor the companies and get the low budget, potentially dodgy, single operators, with just the one cab that they cannot afford to update, off the road.