A mate of mine has been sick at around 11000' twice in the last year, both times he was suffering with a cold from the plane trip. In Nepal we started at around 4000' and went up very gradually.He was very fit (bikes and runs regularly and is only about 35) and drinks regularly from a bladder but he was one of three or four to suffer from altitude sickness, out of 20 or so.None of the locals were sick...
I never experienced that but have not been over 12000'. I think that the others were not used to that sort of hiking.
Franco