If meat from an older animal is tougher, why is it usually the most desirable selection for braising and stewing?

Slow moist-heat cooking methods like braising and stewing tenderize even a somewhat tough meat by breaking down most of its connective tissue. Thus, the original toughness of the raw meat is not a major factor when using such methods. The difference is the flavor of meat from an older animal, however, can be decisive. Since it is generally more flavorful, it should normally be the first choice for moist-heat cooking.