Do you have an especially heightened sense of taste? Perhaps you're a supertaster.
Scientists don't fully understand what causes supertasters to have the function that they do, but what they do know is that women are more likely to be supertasters than men. Supertasting is most likely a genetic condition. Supertasters have more fungiform papillae than regular tasters; these are the bumps on the tongue that help detect the five taste senses (umami, sweet, sour, salty, and bitter.)
Since there is an increased sense of taste sensations such as salty and bitter, supertasters find it difficult to enjoy foods such as broccoli, coffee, brussels sprouts, grapefruit juice, olives, and green tea. Some supertasters are perceived as picky eaters, and this is an unfair characterization. Most supertasters find it difficult to eat vegetables due to the overwhelming sensitivity of bitterness in vegetables that they experience. Highly sensitive supertasters are more likely to dislike a wider number of foods.
So perhaps that fussy adult that doesn't like vegetables is simply unable to enjoy them because they taste overly bitter to them.
(Image: Society for Neuroscience)