Catacombs was designed so you can go from 1 to 50 in it. The mobs start out at level 1 and get higher up the deeper you go, so good for any toon of any level, plenty of quests for whatever level you are also.
The entrance to Catacombs is inside the capital city of whatever realm you are (Jordheim, Tir Na Nog, Camelot). If you specify what realm, I can give more precise location inside city if you can't find it yourself.
good luck