From a scientific point of view:

Taste buds and the cells that make up them die and are replaced just like other cells, so what we physically sense will change.

Also, children's taste buds are designed to like sweeter, higher caloric, higher fat foods as a parts of nature's primitive way to ensure they will eat foods that will increase their chances of survival.

As the body ages and needs quality nutrients instead of just quantity, vegetables do become more palatable.