While there are many causes for childhood constipation, withholding due to anxiety is undeniably a leading culprit. In my opinion, as a parent, fear - due to a lack of understanding of digestive processes - heavily contributes to this anxiety.
Some people have adverse reactions to consumption of short chain carbohydrates. These adverse reactions are not a consequence of malabsorption of short chain carbohydrates. They result from the response of the body, in some people, to that malabsorption.
When we are young, we may think whatever we experience is the norm (we continue to make these types of assumptions in adulthood). I don't know when I became irregular, perhaps I always was.