What levels are the DLCs?

I've restarted playing and can't find my saves anywhere. I have never played any of the DLCs. Are the DLCs scaling? is there a max level to enjoy? I'm playing through from level 1, at 12 now.

Where can I find level range details on the new areas since I can warp to them with the travel system already?

I suggest you to replay the whole mainline story maybe with a different class to see the game from another perspective. Instead if you don't want to replay all, you can try to download some savegames around. If you need I can store online my current savegame of a 40 level Lilith.

The Secret Armory of General Knoxx

Minimum Level Requirement: 51. But I personally tried to start it around level 37 and I didn't found any major problem. It is tough in some parts and you have to pay attention, but you will level up very fast and enjoy the DLC.

The Zombie Island of Dr. Ned

Minimum Level Requirement: 10. Enemies scale according to character's current story progress but do not scale above level 50

Mad Moxxi's Underdome Riot

There is not minimum level requirement for Moxxi DLC, but it is suggested a level 13/14 or more.

Claptrap's New Robot Revolution

I didn't buy it yet but from what I am reading around it should be have a levelling algorithm similar to other DLC.

Playthrough I

Borderlands

I normally beat the game and all quests beat level 33-36.

I've not tried the DLC in playthrough 1 yet, so I don't know what the level ranges are. The below level ranges are for Playthrough 2:

Playthrough II

Borderlands - Island of Dr. Ned

Enemies start at Level 42.

Secret Armory of General Knoxx

Enemies start at Level 51-53. and go up to 58-60 when you beat the Playthrough (known as Playthrough 2.5)

Claptrap's New Robot Revolution

Enemies start between 54-57.

With the advent of patch 1.41, all enemies level up to your current level +1 (or -1, depending) until level 69 (with DLC-General Knoxx's Armory installed) or level 58 (without General Knoxx's armory).

Crawmerax levels up to 3 levels above the hosting player.

With the 1.5 patch, which is due this month (10/10) the content should scale--so that its always level appropriate (though I don't know is that is true both on the up and down sides).

It will make finishing the P2 of Borderlands (along with the P1 play through of General Knoxx, and the P2 play through of Dr. Ned's and Claptrap's) a lot less boring at 61.

Max level will be increased by 8 for everyone too (to 58 to those without Knoxx and 69 for those with) with the same patch.

I started Island of Dr. Ned at around level 15 and found it relatively easy.

I tried The Secret Armory of General Knoxx at level 27 and I got roasted... playing now at level 37 and it is tough but doable.

I also tried Claptrap's New Robot Revolution at level 27 and did not make much headway there either... Have not tried again yet.