There is some evidence that small rotator cuff tears of a single tendon can heal on its own. Age may be helpful in terms of good blood supply but most younger adults are also very active. Increased activity is more likely a deterrent to healing in this case.
But the biggest determining factor is the size of the tear. Tears less than 100 mm2 (less than two and a half inches) may heal. MRIS may not be able to show a tissue defect but the new collagen tissue has reduced strength. There is always a risk of rerupture for the active, young adult.
For moderate to large sized rotator cuff tears, repair is advised. Even if it reruptures, the results are usually better than if no repair was done. And in the long run, repairing the rotator cuff seems to offer some protection to the joint from degenerative arthritic changes.