All enemies outside the cities and some inside will level as you level. For example, if you are at level 10 and are fighting Boars outside the city, when you reach level 15 you will be fighting a Troll. The same goes for missions such as the Forlorn Watchman. If you do the quest at level 10, you will fight Ghosts on the ship. If you do it at level 20, you will fight Wraiths. Do not waste time trying to level up just to beat one opponent in a quest because they could be set to always be three levels above you. This same style of progression also works with armor and weapons:

Armor

Fur/Iron: Level 1
Leather/Steel: Level 13
Chain/Dwarven: Level 16
Mithril/Orchish: Level 110
Elven/Ebony: Level 115
Glass/Daedric: Level 120
Weapons

Iron: Level 11
Steel: Level 12
Silver: Level 14
Dwarven: Level 16
Elven: Level 19
Glass: Level 112
Ebony: Level 116
Daedric: Level 120

Note: The armor and weapons usually do not appear at traders until you are two levels above them, sometimes longer. Also, any "secret" weapons such as the Umbra and the NPCs with them or wearing the "secret" armor are level locked. They will not get any more difficult because usually their levels are already about 22 to 30. However, as you become better with a blade (in the Umbra's case), the better the attack will be.