I don't they are referring to DS patients. Just that Darwinian evolutionary processes only selects for traits that make an impact prior to reproductive age. I

I don't they are referring to DS patients. Just that Darwinian evolutionary processes only selects for traits that make an impact prior to reproductive age. If a trait isn't expressed until after you have kids (MS, Huntington's etc.) it becomes easier to pass it on. Also as more diseases become survivable due to treatments / science then it would be easier to pass those traits on as well.