Re: Who's eligible to teach Aikido? And what level a sensei should be?
The Aikikai gives minimum ranks for all three levels of certified instructors. Fukushidoin are 2nd or 3rd dan, shidoin are 4th dan or above and shihan are at least 6th dan.
So to be a certified instructor one would have to be at least nidan. There are however many circumstances that could lead to someone of lower rank instructing. Personnally I would avoid learning from a low ranked instructor unless they had a good active relationship with someone of higher, preferably shihan level ability. Even with high ranked instructors (lets say the 5th and 6th dan shidoin and the shihan), I consider it a good sign if they are actively still going out to seminars to interact with others of greater or equal rank. It's a good sign that they are still keeping their aikido "fresh" and working to improve.