Uh, ages ago. I'm fairly sure that killing or abandoning the disabled has been a part of human culture for far more time and far more culture than it hasn't been a part of it. Not killing inconvenient children after they are born strikes me as a very modern thing. And continuing to not kill them if they have disabilities is really not the norm. There are countless examples of the killing of people with issues. From the obvious extremes of Nazi Germany, to the day to day smothering of extra infants in Europe in the Middle Ages, to the abandonment of the deformed by the Greek who saw it as a moral imperative and that to raise a child with a deformity was to give in to weakness and evil. Abadonment and killing of the elderly who are now too weak to be productive was so common that the theme is considered a major category for fairy tales, and you can find a dozen different stories with it as a key element.

Not that I agree with it. I'd have been killed multi[ple times over by Nazi Germany standards and am missing large pieces of my family tree because of them, so it hits a bit close to home. But mankind is not so close to civilized as you seem to think, and our recent attempts to become so are recent and just small steps in the right direction. It's not surprising that we're not doing that well yet.