OK, so if we accept that something that is a neutral term can turn offensive (retarded) and that we should stop using it as a result... why not do it for other terms that have the same problem?
Right around 1990, and I don't think the "really, don't say 'retard'" thing took strong effect until early 2000's.
Our culture's imperfect, but there's a fair bit of evidence that it's a kinder, more courteous world-- especially for youth-- than 30 years ago. Taking some sharp edges off language may have helped with that.