Just for the record, young grasshopper...
Warriors and paladins can wear mail or less up to level 40, and at 40 they get the ability to wear plate.
Shaman and hunters can wear leather or less up to level 40, and at 40 they get the ability to wear mail.
Druids and rogues can wear leather or less up to level 40, and at 40...nothing changes.
Warlocks, mages, and priests can wear cloth or less (naked, much?) up to level 40, and at 40...nothing changes.
Death Knights start out at level 55, and I'm pretty sure they can wear plate from the get go, but I haven't played one and haven't read up on them much at all.
All clear? Enjoy!