I don’t really agree with the OP that AC age or type has a big effect on number of pax they will get.
Type does have a bigger impact than age.
E.g. I mr pax don’t want to fly for more than about 1 hour in a prop or other similar small A/C like the Embraers.
Why? The seats are even smaller than a standard economy, so it is very uncomfortable.
The props are noisy.
There are many airlines today (2013), who operate MD-80, DC-10, B737-200, A320, Fokker 80, Fokker 40, BAE-146 which are 30+ years old.
North West Airlines now Delta has many of these in the US.
In Africa there are also more examples of these.
In South Africa some of the budget airlines use the older A/C.
In South Africa and in Australia I know of a few tourist operators who specifically use very old planes like DC-3 for example for the nostalgic and tourism purpose.
People pay a high price to go on these yester year planes.
Until only 5 years ago Aeroflot and other Russian and Ukrainian airlines used TU-134, TU-154 A/C extensively which were 30+ years old.
They changed not because these A/C were unsafe, but because of costs to operate them (fuel burn, maint, and 2+ number of pilots required) and the new noise restriction at many airports.
Most people flying care about 1stly the price and safety reputation of the Airline then about service.
Most don’t give a hoot about what A/C they are flying and how old it is, as long as it is comfortable, service is good, and they are pretty sure the engine or the wings won’t come off any minute.
Note I said safety reputation of the Airline not a specific A/C, and that has to do with how well their A/C is maintained, and how well the pilots operate these without issues.
If engines fall off and tyres blow out and causes an emergency, the said customers on board will not likely fly again with that airline, unless there is a good explanation. These events also get lots of news publicity and new customers will think twice before flying with that airline.
Quantas, Air France and Emerites for example in the last 2 years has their reputation tarnished because of the A380.
There were engine issues and airframe issues, and recalls by the manufacturer. Yes these are brand NEW A/C.
Quantas moved their maint to China and now they have more maint related emergencies, they are now considering moving maint back to Australia.
Both AF and QF, had in the last 10 years issues with fairly new A300 type aircraft (not sure the specific model), where their computer systems have issues, electrics or electronics go on fire, hydraulic failures, etc.
From a pax perspective, the issue is not the age of the A/C, it is about how well maintained it is, how safely the pilots operate it and how reliable the actual A/C is.
From an airline perspective, they care about cost and reliability and commonality to minimise training and maint.
Airbus is preferred simply because of commonality in the cabin for the pilots between types, so training is cheaper for example.
Both Airbus and Boeing for example offer the same airframe but different engine options, because for maint purposes, the Airline will try to use the same engine or same engine manufacturer for all their A/C.
E.g. Airline X might have a combination of Boeing and Airbus, but they are all GE engines to safe on maint costs.
And in fact the most expensive part of maint for the airline is the maint on the engines so commonality and special deals with engine manufacturers and maint contractors play a big role in saving money for the airline.
Even the new TU-204 now offers different engines including RR, GE and PW over and above its PS engines.
Of course some times the airline need to overhaul the whole A/C to restore both engines and airframe to a almost new condition, which I believe is very expensive too, and I think is called a D check.
Obviously most airlines will probably want to avoid the D check if they can and either return the lease or sell the A/C before then.
So what I am saying is that there is different factors why an Airline might not want to operate and A/C which as very little to do with the customer’s perception or affects the ticket sales.
For ticket sales customers mainly care about the price of the ticket and the class of service offered for that price.
The cheaper the price for a good destination the more tickets will be sold.
Obviously it is up to the airline to figure out if they can offer tickets at that price given their costs.
The game obviously tries to simulate some aspects of running an airline, but by no means does it simulate the exact conditions and things airlines need to contend with.
E.g. real airlines have different competitors some are private or public companies, some are state owned or semi state owned.
The state owned ones obviously can service non profitable routes or heavily discount the ticket as their objective is not always to make money, but to provide a service to the people or to provide jobs and indirectly feed back into the economy.