| When you can join is not determined by your level, but by your reputation. I see no harm in joining early, because, since almost no (if any, please remind me if I'm wrong) quests have a time limit, you can always wait until later if it's too challenging. And in my experience, the only thing that is barred from you once you have joined a house, is joining another house. I always join a house as soon as I am able, but then again, I rarely play Telvanni.
What mod is it you are referring to? |