Quest minimum level

Update 18 silently added hard lower limits for character level to be able to enter a quest. See Quests by level for what quests are each level.

The same restrictions apply to Wilderness Adventure Areas. It does not apply to Challenges which instead have specific minimum levels for each challenge (the lowest selectable level for the challenge is also the minimum level which characters can enter it, and characters of that level or higher can enter at any selectable level).

For Heroic quests (quests with base quest level 1 to 20) the minimum character level that can enter is determined by the quest's base level (difficulty selected does not matter). For example, The Pit has a base quest level of 7, so the minimum character level that can enter the quest is 4 (according to the table below); characters of level 4 or higher can enter this quest on any difficulty setting.

For Epic quests (quests with base quest level of 21+) the minimum character level that can enter is the quest's effective level (difficulty selected does matter) - 4 (or level 20 if the formula would allow lower). For example, Caught in the Web has a quest level of 24 on Epic Normal, so the minimum character level that can enter the quest is 20 on Epic Casual (Epic quest level 23 - 4 = 19, so use the minimum allowed of 20), 20 on Epic Normal (Epic quest level 24 - 4 = 20), 21 on Epic Hard (Epic quest level 25 - 4 = 21), and 22 on Epic Elite (26 - 4 = 22). Since Update 23, any character of level 20 or higher can pick up and enter any level quest. Chain bestowers, however, will not talk to low-level characters and presumably follow the old level requirements.

Entire Party Rule
For Heroic quests, all members of the party must meet or exceed the quest minimum level for anyone in the party to be able to enter (or re-enter) the quest. For Epic quests, all members of the party must be at least level 20 for anyone in the party to be able to enter the quest. For example: if someone enters the quest and then summons a lower-level hireling (below the quest minimum level), no one else in the party will be able to enter the quest until that hireling is dismissed. After all the party members that need to enter the quest are inside, you may summon the hireling again (or invite other low-level players to the party to chat etc. even-though they cannot enter the quest).

Caveats
Some notable problem quests:
 * The Twilight Forge (base level 11) raid can be started by characters of level 8 but you must be level 9 to continue into The Titan Awakes (base level 12) when you get to the end.
 * Ruins of Gianthold (level 13) wilderness area should allow characters of level 9 but you must be level 10 to enter, so the level 13 quests in the area cannot be entered until level 10 (even though the quests can be acquired at level 9).