the lvl restriction is based on the ancient quest, you cant do it when you're not lvl 20/40/60.... without the quest you cant go down to baal....
BUT if there's someone in your party who gets the baal quest you will get it too, even if you're just standing around in town...
example: you're a lvl 1 char... you can not do ancients and not go to baal norm... but if you join a game, where someone in your party is doing the baal quest and he finishes it, you will get it as well when you're standing around in harogath... so you now finished norm with lvl 1... do this in nm as well and you're a low lvl char in hell