Since your actual time frame is impossible to really give a definitive answer. I'll explain this.

1-Autism is the diagnosis du jour. Like ADHD back in the 90's if you kid is a bit different they get slapped with an Autism diagnosis. The early 1940's are when the diagnosis was first made. We speculate that many of our most celebrated scientists likely had some form of very high functioning autism. People with sterotypical severe autism likely wouldn't have survived past early childhood.

2-Down's syndrome has quite a few severe medical complications, including heart issues, digestive track, inability to consume enough calories. While a child that is less affect may have lived into adulthood, most children would have died in infancy to early childhood.