I would say that if your Discipline, Conviction, and Lore sum to 9 you qualify (with none lower than Fair (+2) on its own).
In terms of special powers, you have to have Either Evocation or Thaumaturgy (and at least Channeling or Ritualist in the one you don't have), as well as Wizard's Constitution, The Sight, and Soulgaze.
This is why Dresden is such a scary person. He is in his mid thirties (35? 36?) and he is really strong (he had Conviction 5, Discipline 3, and Lore 3 at age 25!).