Players and NPC's always the same level

[N.Gamma]

Is there a mod that makes players and NPC's always have the same level, so it doesn't matter when you go where in the game?

[Antiope_Apollonia]

I've never come across such a mod, but it wouldn't be very difficult for you to make one yourself.  You'd basically just need to edit two variables in each NPC's record, which could be achieved with a quite simple xEdit script.  NPCs have min/max level variables, so set them to 0/999 respectively.  And there's a flag for "levels with player" that you'd need to enable.  That's all there is to it.  You could even do it by hand, but it'd be a murderously painstaking process to do manually given the sheer number of NPCs in Skyrim.