This quest doesn't have a level requirement per se, but in order to finish some of the areas it's required to have a minimum level.
The 11 easiest areas are:
- Ab'Dendriel
- Carlin
- Thais
- Venore
- Kazordoon
- Northern Darama
- Edron
- Feyrist
- Krailos
- Quirefang
- Hrodmir
I was able to complete the areas above at lvl 112 playing solo.
The next easiest ones are:
- Yalahar: possibly will have to face plaguesmith, destroyer, hellspawn and a couple of other stronger creatures but other than that it's fine.
- Marapur: You need to partially complete the quest Within the Tides in order to gain access to Marapur, and this quest requires going to Asura Palace and killing a few bosses. Could be done earlier, but recommended for lvl 200+. Other than that, after you get access to Marapur, the actual discovery process is very easy because all the subareas are surface-only (no need to go into any cave).
- Tiquanda: Oskayaat, Asura Palace and Medusa Tower could bring some trouble. Other subareas are easy.
- Southern Darama: Tombs require some effort but it's doable. Cobra Bastion requires lvl 250 to enter. Other subareas are easy.
- Vandura: Kharos is a bit difficult. Bounac requires lvl 250. Other subareas are easy.
- Zao: Razachai is a bit difficult but doable (specially if you have friends to help). Falcon Bastion requires lvl 250 to enter. Other subareas are easy.
- Oramond: You need to partially complete the Dark Trails Quest in order to access Oramond Mountain Hideout. Other subareas are easy.
The two last and most difficult areas are:
- Roshamuul
- Kilmaresh