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.