There are certain rules in place to prevent people from powerlevelling on higher and lower level monsters, to this end you only get experience from killing monsters in a + or - 10 level range of your current level.
You have to remember that some types of monsters like animals and fish are much weaker than others, so if such a rule was not in place people would be able to freely powerlevel on the weaker critters by killing much higher level monsters.
You are still able to kill higher level things for the loot they drop and bragging rights, but you gain no experience from doing so.