It is difficult to estimate how long a particular airframe might last. Also, different aircraft might last different amounts of time for esoteric reasons. For example, mercury amalgamates with aluminum and can weaken it. Even a small amount of mercury can significantly weaken an aluminum structural member. Therefore, you might have one plane with some mercury contamination that will not last nearly as long as one free from that accidental contamination.
Also, how the aircraft is used will significantly affect its lifetime. A few high-stress maneuvers can weaken an airframe. Conversely, an aircraft that is babied and lightly driven may last for many decades.
Ultimately, there is no way to predict how long a given aircraft will last. The best thing to do is to observe it and when cracks start to appear, then you know there is a problem.
To compensate for fatigued structural members, it may be possible, but it depends entirely on the scope and nature of the fatigue. If the fatigue is limited to a restricted location, then theoretically that area could be reinforced. The problem is that the complexity of the engineering is such that in most cases it will be cheaper and more effective to simply replace the aircraft.
Usually wear is most severe at the wing roots, so a plane that is experiencing fatigue can potentially be improved by replacing the wings, but the cost of this is pretty significant, so normally you would only see this in an air force that could not buy new aircraft for some reason.