I am on my first play thru of Skyrim. I know in certain parts of the game the mob difficulty scales up to your level. I was curious to know if this applied to the main quest line as well as the faction quest lines. Right now I am level 37. If I focused strictly on the main quest line, could I finish it over the next several levels or do I need to be near the level cap in order to be strong enough to complete the quest?

Thanks

There is no cap, it can be done at any level, no penalties.

However, you might find a lot more Deathlords coming your way though.

And a Dragon Priest, regardless of level...

I encountered them at level 32. I only survived by running around and shouting them off cliffs.

Depends on what difficulty you have it set to...higher difficulty would require higher skills and such but the MQ isn't that hard overall, the Solstheim quests are a different matter. Did you hear about how they had a MQ speed run contest in Bethesda before release? 2 in house guys completed it in roughly 2 hours on the default difficulty without leveling up at all as that slows you down to do so.

I usually tend to prefer to not delve to deeply into the MQ until I've leveled at least past 20 so you've nothing to worry about. As others mentioned you can start it whenever you want you will just run into the tougher enemies at higher levels but also better gear and loots.