When does Steel Plate Armor begin appearing? On UESP it says level 18, but I've had it appear before that, so I don't think that's right.

When is the correct level?

While I;m here, what is the level for other items to appear (Orcish, Dwarven, Daedric, etc)? I want to know cause I often buy or loot my armor, I'd rather not spend points to make armor.