The latter is easy: no. I'd object to using a magic Down's syndrome wand to intentionally create a Down's syndrome fetus, but if the fetus already has the syndrome, then that's a pre-existing condition.

The former is, in that specific case, also easy - no, it shouldn't be illegal either - the risk isn't that large. (my own mom had me after age 40). However, see my answer to firstfrost below about there not being a clear bright line - as the probability of a behavior causes defects increases, and the severity of the defect increases, the justification for intervention becomes higher. I realize that some might argue that even behavior with 100% of causing a severe defect should be allowed, either because of right over your own body arguments or libertarian arguments, but my opinion differs...