Yes but we're all different with unique histories. I would have thought it's down to certain things like whether someone was breastfed, the health of the mother pre and during pregnancy and during breastfeeding period, whether she'd had other children and if there wasn't a long enough gap for her to build up her reserves before having another child, as pregnancy can take so much out of a woman. Her essential fatty acid profile...you can't pass on to your children what you haven't got yourself. EFA profile of father and grandparents.
Then the other classic reasons on top of the above, like overuse of Antibiotics , poor diet, stress, vaccinations.
The ones who seem fine may not be fine...after all we don't just get ill on the day we get ill do we, they may develop something else later on in life even if they seem to be able to tolerate taking Antibiotics earlier on in life.
Anne