I got asked for advice tonight by someone on how to be an instructor in magic for someone that is just starting out. The person that asked me was twenty. My advice: don't.
This has nothing to do with the individual asking me. Unless you spent your teen years shut away in the Black School under the Glaciers of Iceland, or were raised as a Eckankarian Tulku in the secret Katsupari Monestary than you are simply not ready to teach magic by the ripe old age of twenty.
You can and should recommend books and give advice. This should be done from the perspective of a fellow student of a higher grade giving advice to a younger student. Actually developing a curriculum and teaching is a whole other ball of wax. This leaves the question though: when should we teach?
If you belong to a solid tradition, than there are probably some rules in place. However, even in the oldest traditions there can be some wiggle room. For instance, you should not assume that everyone who calls themselves a Lama, or even is recognized as a Lama by the establishment, has been on three year retreat, or any major retreat at all for that matter.
Outside of those traditions magic is basically the wild wild west. Thats fine. I like it that way.
My advice however is not to teach when you think you ready.
When others think you are ready to teach, I still don't think you should jump into it. Instead you should maybe be in informal mentor. Perhaps lead a group of peers. Get some experience advising.
Only when others have been asking you for a while, and you have some experience of advising peers and mentoring should you consider setting yourself up with students.
This is just my advice of course. There are no hard rules. You can do as you please. Just be prepared to not be taken seriously if you are 20 and your students are 18.
Unless of course you were raised in the Black School or Katsupari Monestary. In which case: can I be your student?