After doing some reading on various message boards and testing it for myself, it seems like the stats (skills) of followers are determined when you first get them, and do not level with the player. So, as the player levels up and encounters higher leveled monsters, the follower still has the same stats as before, becoming weaker relatively.
E.g. if the player gets Lydia at level 5 and then levels to 50, his Lydia will be significantly weaker than another player who gets Lydia at level 50.
It doesn't really make sense design wise. If someone wants to use a follower for the entire game they should not feel gimped because they got the follower early on.
An example from another forum:
Did the kill/respawn technique with Uthgerd, who would've spawned around my early levels, and I'm 28 now
Uthgerd 1 > 2
Health: 108 > 365
Stamina: 67 > 140
Heavy Armor: 26 > 74
Two Handed: 29 > 45
Huge difference on the whole. Seems like a pretty big design oversight. The only reason I can think that they did it this way was to urge the player to go out and keep finding new companions, but that seems to defeat much of the purpose of games like ES that are based around choosing your own path and customizing your world to your desires.
Uthgerd 1 > 2
Health: 108 > 365
Stamina: 67 > 140
Heavy Armor: 26 > 74
Two Handed: 29 > 45
Huge difference on the whole. Seems like a pretty big design oversight. The only reason I can think that they did it this way was to urge the player to go out and keep finding new companions, but that seems to defeat much of the purpose of games like ES that are based around choosing your own path and customizing your world to your desires.