I stumbled across this Evolutionary Psychology article on Disgust, which seeks to disprove the notion that disgust is there to prevent the eating of pathogens.
The first example to illustrate disgust is eating excrement. Another frequently cited example in other articles (cf. Freud’s Totem and Taboo) is incest. What both of these have in common is the mingling of categorical bodily opposites. In both of these examples, something has to go back into a body it came out of, to reverse the order.
If you grew up kosher, you can feel disgust at seeing someone eat a hamburger while drinking a glass of milk.
These cases all have in common the mingling of categories — Going Out and Coming In, Milk and Meat. And in the latter case, Milk comes out of Meat, which is also a kind of violation of Going out and Coming In.
To be able to feel disgust, I conjecture, you must be first be able to understand categories, and that doesn’t happen before a certain age.