So, I was chugging along and doing a few main story quests, and I noticed one was level 15. I started it, and looked down and realized I was but level 14! And I continued through a couple more level 15 quests (and then finally leveled up to 15).
Just figured I'd pass it along. My wife, who was 'right behind me' was unable to start those same level 15 quests until she leveled up to 15. Maybe it was just a fluke?