From Official Site
Dragon Age Origins has the most story and gameplay content in any of our games since Baldur's Gate 2. As with all BioWare games, the final level your character will reach will depend on how much of the story you choose to experience. If you only do the core story, what we call the critical path, you will be at a lower level than someone else who does the critical path and many of the option side quests in the game. We expect most players to be between level 15 and 20 at the end of the game.