That would be the level recommended by the people who typed in the quest to put here. You can get missions at any level; they are dependent entirely on rank. Whether or not it is possible for you to actually do them is up to you. I helped a level 14 red mage kill the dragon in Giddeus. He got the mission at level 11 - no subjob, 11 was all he had - because he had the required rank.
He was lucky to have a group of much higher level people to pull him through the fight. That was his mission 2-3.
I have Sandorian mission 3-3 ready to claim at level 12. The requirement listed is 35, but I can get it.
Quests can depend on level, fame, doing previous quests, etc.
Allakhazam's is not an official source of information on the game, until this game has been played in the US for a year or two, I wouldn't count on this being terribly accurate.
If you can't get a mission (as opposed to quest), increase your rank until you can. Give crystals to the R.K, I.K, or W.W. guards (depending where you live), or redo earlier missions.