There must be some selection effect at work. "Those who live with them" are the ones who have chosen to live with them, which excludes:

a) all who have learnt about the situation before birth and chose abortion,

b) all who gave the kid away to some institution.

A N == 1 case from my life. My classmate had a Down kid at 20 - very rare, as Down is not typical in young mothers. She seems to be happy, even though she sacrificed her dream of a bigger family for him; it was so challenging having a Down kid that she didn't have any other.

But the father absconded and wants nothing to have with his disabled son.