Passengers will flag down any old bus going along the road with scarcely any regard for where it is going or whether it is even operating a public service!
It's been my experience that passengers notice the veteran buses when their route gets a lot of them; I have been on services in Perth that copped a lot of the last high floors and overheard remarks from passengers who were frustrated they always seem to end up on the 'old rattly buses'. Without such a sharp distinction between old and new in the fleet anymore, those remarks are nonexistent now; although passengers are accustomed to having USB charging points in enough buses that they do tend to notice when on a slightly older vehicle.
Certainly passengers do notice when their bus is untidy (amazing the difference clean wheels, for instance), and in a poor state of repair - little things like working lights, and a tidy bus make all the difference for a passenger's perception of the service, no matter how old the bus.