As McCoy might say, “Different people develop at different rates.” Obviously, whatever it was that proved fatal to other children hadn’t kicked in on Miri and Jahn yet, regardless of how well “developed” they were otherwise.
They were. One was Roddenberry’s own daughter (“Dawn,” IIRC). I think she was the one who went berserk over the tricycle.
I know. (Shatner’s kids were, anyways, according to several of my books.) I was being facetious 10 years ago. 
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Rereads OP. On yeah, we were to discuss the eps as tho we had no other knowledge of Trek stuff. Those were fun threads.
Note Miri was starting to develop the lesions.
Yes, finally. Just before the cure was discovered.