There are multi possibilities:
1) (MPI) : Disable the unit ability at Map Initialisation; enable it when your hero reaches a certain level. This will remove the ability icon from the hero!
2) (MPI) : Create a dummy unit with name "Level 7 Paladin" (or something like that), and add it to the unit ability requirements. When the hero becomes level 7, create a dummy unit for owner of leveling unit.
3) (MUI) : Create a dummy passive unit ability (e.g. based on critical strike) and add it to the hero. When the hero becomes e.g. level 7, replace the passive ability with the real ability. While it's slightly harder to implement than the other 2, I think this is by far the best solution out of these 3.