The difficulty level is determined based on statistical data from 7sage students. The level isn't currently listed at the moment probably because there isn't enough representative data to say what is a 4 star or 5 star question at the moment.
I think you need to have a star to be a Sage. Sages have 170+ (like me--hooray), but also have that track record of posting serviceable information here.
I'm not actually sure what the requirements are to be a sage, but you do indeed receive a star when site staff have observed you to be a very helpful, active, and insightful member on the forums over a period of time.