I think many things affect kids...but they don't have the tools or ability to articulate how they feel until they are adults.

The growing process seems to be overwhelming to some extent and takes lots of energy. Kids and young adults are always busy busy and it isn't until adulthood that they start to monitor internal states.

Also an adult body takes more nutrients to run properly than a small child's body. There are probably other variables at work too, that have just not been identified yet. The medical community still struggles with autism, and as common as it is becoming, that is a real worry IMO.

You bring up good questions. I have met so many people who as adults cannot articulate clearly what hurts or is bothering them. It is pretty common. I recall my husband's old family doctor coming out to me in the waiting room, asking why he was there. (he had just come home from Vietnam, had gray skin and terrible stomach problems, and couldn't explain that to his own doctor! ) I told him about the gall bladder-like symptoms and that hubby was taking those terrible anti-malarial pills, which are quite toxic.