From a strictly mechanical point of view, it's not the number of years, it's the amount of wear and tear on the machine which is proportionate to the amount of media printed, or if you prefer to the amount of ink laid down. Like mileage on cars.
If you rely on your machine for a living, be sure to replace it well ahead of any serious down time, which can be a lot more expensive than simply buying a new machine now and then. My wife's lightly-used 2400 is still running great at 7 years, but in that same time I have driven three wide format printers into the early stages of developing problems.