Like a water heater, a well water pressure tank should last for a decade, if not longer. It is not an appliance that requires frequent replacing. Given enough time and use (and it gets constant use), it will need to be replaced. That being said, water pressure tank repair may be a possibility as well.
Time for Replacement
A well water pressure tank in need of replacement probably looks its age. The paint on the outside may be flaking away. If you have a "captive air" tank with a rubber bladder, you might eventually detect small particles in tap water. Failing water pressure may indicate loss of air pressure in the tank, another signal it might be time for a replacement.