How important are quests in the younger years? I can see them being super valuable to a mage class who would want to level from good exp gains and little risk to life, but a sparring class?

For example logically I would reject Simon's quest and focus on gaining experience from fighting as this would have the best chance of improving my skills, something that questing doesnt really do (as far as I know!) in the younger years.

So how important are quests for a young sparrer? Do Imms favour questors? Do they make getting edges easier? Or can they be ignored for more hands-on learning?

I would suggest doing all of the newbie quests you can for the bonus exploration and commerce experience. Also, you get some nice items depending on the quest in question.

For the most part, do what you feel like. In the long run you probably aren't going to see much advantage or disadvantage either way.