hi,
you both may be right. I have come across in dealin with the problem of impingment too often. I have seen varying causes of the problems.
1- age:
it is more common in people with age more than fifty. the impingement start due to the poor psoture patern as the people in in this age group have a tendency of increase thoracic lordosis, decrease proprioception and delayed recruitment of the of the musle fibres.

2- it is common in the middle age population as well. During my clincal experience i have found that those people who are working more in the middle range of shoulder felxion, develop the problem. e.g. those who are frequently involved in jobs like putttings things in Shelves, puthing soaked cloths in the wires over head, throwing balls in the middle range as in cricked etc.
the supraspinatus impingement can be confirmed by applying moderate dose of ultrasound with out moving the head(transjucer) on the to of the shouder (supraspinatus proint), placing the arm in behind the scapula in medial rotation. the patient complain of shooting pain after a minte of constant application. I found this working in the other tendenitis problems as well where i doubt any other cause of the problem.

ther are of course other khown causes as well