My First Raid ever was Zul'gurub , we killed Venoxis in about 1 Hour and then wiped our asses off with Jeklik. I think like 4 Hours or so XD.
But i dont count the 10 or 15Man Dungeons. That would be Scholomance then , or Ubrs? I don't remember anymore
We had low lvl paladins with us in the early Bc Days (Horde ) , and i know that i was in a McAlt Raid with my lvl 57 Warlock looting t1 for leveling XD.