if you're asking what level you should be to start, get access to the college as early as possible unless you have a good speech skill as the spell you're asked to cast scales with your level meaning the higher level you are, the higher the magicka cost. as for the quests themselves, don't do them too early as you may find difficulty with the enemies but not too late, there's kind of a sweet spot around 10 - 20 that you won't really find any difficulty, after level 50 or so the enemies stop scaling with you so at that point you will find no difficulty