I think it can be genetic from the ram but stand corrected if that's different. Yes it can cause blindness if not dealt with promptly- 1ml of pen strep into each eyelid will puff them out and stop them from turning in. It can be really nasty, normally needs some cream into the eye to treat any ulceration too. We had a little dorset x lamb with it this year in both eyes, she went off her milk because her eyes were so sore so it really needs addressing asap. Not sure about older sheep- are you confusing it with pink eye?