Even healthy trees can be prone to limb failure, this is often related to poor branch structure, rather than disease or age of the tree.  For instance, branch unions that have a "V" shaped crotch, or the presence of a bark inclusion are more likely to fail.  This is due to parallel wood fibers, much like the easy ability to separate chopsticks. Installing supplementary support systems such as brace rods and/or tree cables will help reduce the likelihood of stem failure in these situations.

 

Strong branch unions are often "U" shaped and are less likely to fail, think of a chicken's wishbone, the fibers are well integrated. Structurally pruning young trees, or performing reduction pruning on select limbs is typically done to prevent the need for supplementary support. 

 

More information on structural pruning can be found on the University of Florida website using the link below.. Link: https://hort.ifas.ufl.edu/woody/structural-pruning-flash.shtml