At what level should one try to teach go?
You can start to teach beginners as soon as you develop a basic grasp of Go, which I'd say is typically around 15-20kyu. Do not be surprised if your students surpass you quickly if you start teaching at that level, though.

Generally, people feel comfortable teaching players whom they can beat comfortably and consistently. But this is not necessarily the case. Some of the older players serve as teachers to younger players who are actually stronger than they are--this is fine as long as the difference is due to readings skills, which cannot really be taught.

Remember, players of any rank can give students the wrong advice. After all, nobody knows the solution to Go, and even the strongest professional players frequently disagree with each other on the appropriateness of moves.