All machines eventually will develop cracks, when and where depends on many factors. Some are caused by factory defects, these type will probably show up sooner rather than later.
The type of material the machine has been used in is plays a role as well. Digging in rock obviously is harder on a machine than bailing nice soft fill. The operator of the machine can also influence how quickly fatigue starts to show.
Experience has shown me over the years that no matter how solid a piece of metal looks, it can crack. Cracks can be repaired easily depending on where they occur and if they are spotted early. By early I mean before any deflection or bending happens. Inspect your piece of equipment on a regular basis, particularly around welds and areas where the boom or arm is intersected by pins.
The repairing of cracks should be undertaken by a person experienced in this type of repair. If done incorrectly the crack will most likely reappear. It is not simply a matter of applying a plate and welding it on. The right type of steel must be used depending on what the original structure was made with. The repair under the plate must be done correctly as well or the crack will reappear. Also the plate must be shaped in such a way to avoid further cracking due to it being welded onto the original part.
Cracks will happen, I have seen machines with less than 3000 thousand hours come apart, the point being that new does not make the machine immune. A good time to inspect your machine is when your greasing it. A crack found early on is far easier to repair, your mechanic will appreciate your diligence in making their job easier and avoiding a potentially huge repair.