[More like thirteen, but Nico does look a little older because of how gaunt he is and how serious he can be.]

They can't be used against mortals, pass right through them like air. At least Celestial Bronze does, but I'm sure the others do too.

Imperial gold is... what's the word. Volatile? It can blow up. Only Artemis' hunters can use Lunar Silver and it's good against werewolves. Celestial Bronze is kinda the most basic of the bunch. Stygian Iron can only be used by those of Chthonic origins, and it absorbs the essence of monsters, so it takes them longer to regenerate in Tartarus than if they're killed by the others.