When do you allow your pre schooler to experience junk food?

As much as possible I wouldn't introduce junk food to pre schoolers. I would want my children to grow up on food that is nutritious and healthy, want them to get used to eating vegetables without bribing or forcing them to eat it and that can only happen if junk food is not at all in the food plan. But of course we can't really avoid them eating it, especially when we're not with them. However, if you must, let them eat it only on special occasions.