If they are sentient it's entirely possible that the power crystals do not gain in personality because of age but are instead crystallized forms of....something. however you look at it if this world were earth 20000 years from now perhaps some gods of the dresden verse hid parts of themselves in crystals when they saw what was coming, perhaps 20000 years post Alera we are seeing what became of furies, perhaps predator was a great fury, or maybe Artemis or Sekmet while mistshark was a dark god and the Itasca power core came from the three graces or furies. If they are sentient they could even work well together or not so much depending on who they are. I'm just inverting what yall are saying somewhat, the age of the crystal doesn't effect the personality so much as the personality is what chooses to create the crystal. We have seen that bits of human souls can be broken away into books so why not natural or godlike powers putting pieces of themselves into the power crystals, maybe they even cut a deal to do so.
