Go to Advanced ---> Gameplay Constants, scroll all the way down to Hero XP Gained - Creep Reduction Table and it'll have the values 80, 70, 60, 50, 0, that means at each respective level you only gain 80/70/60/50/0% exprience from the creeps, delete the rest of the values and put 100 in and all creeps will give full experience to all levels of heroes.
I also have a very simple exp trigger on hand that uses hero and unit levels to adjust precentages to fit the hero level (i.e. a level 1 hero killing a level 4 creep would get 4x the exp a level 4 hero would, but reversed a level 4 hero would only get .25x the exp a level 1 would get for killing a level 1 creep) if you would like to test it out to see.
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