36% of youth in 2009 reported having heard the word "retard" directed towards someone with an intellectual disability.
My wife's brother had a birth injury and endured it pretty constantly throughout his school years.
And given that there's people with that experience, perhaps it's time to heavily discourage use of the word in general. Calling your buddy it to be funny helps normalize it for the people who want to use it to be cruel, and gives them cover.
Haven't we been doing that in the US since 1990?
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.