How common are rotator cuff tears in nonathletes?

Common. It is fairly common to get a rotator cuff tear. Some tears are clearly from injury, but there can also be an arthritic impingement of the rotator cuff that can lead to a rotator cuff tear in the older population as well.
Rototator cuff tear. Incidence increase mostly with AGE. Forget about the athlete or the supposed work related activity, AGE is the most coon factor in rotator cuff tears, , many are degenerative, and many much older patients don't get them repaired as functional demand drops.
Very common. The incidence of full thickness rotator cuff tears increases with increasing age regardless of whether they are involved in sports or not. The prevalence of rotator cuff tears in asymptomatic people is approximatley 0% for age 40-49yrs old; 10% for 50-59; 20% for 60-69; 40.7% for >70yrs/old.
Common. More common than in athletes. The average age of a rotator cuff that that requires surgery is 58. Young athletes are the exception, rather than the rule. They have from falls, heavy labor, wear and tear, etc.
Common. Rotator cuff tears are common in all walks of life. Impingement on the acromion probably contributes to some tears. The most important factor, however, is probably the normal aging/ tendon degeneration process. In fact, as many as 85% of all patients with a known tear have no recollection of an injury that led to the tear.