Re: Who's eligible to teach Aikido? And what level a sensei should be?
Rank is largely subjective. Criteria can vary greatly within a single organization, and even more so between different organizations. And the ability to teach is not a function of rank, although the authority to grade others certainly is.
I have encountered many lower dan grades in my travels, and even some kyu grades, who could teach circles around many high-ranking Aikidoka. The ability to connect and communicate what you know is more of a personal skill than a result of training. Some seem to be born with it, others will never get there.
And the attitude of the student toward learning is also a factor. Good students can go a long way toward bridging a gap.
My personal feeling is that any time two people interact, regardless of their differences, each has something to learn from the other, and each has his own contribution to make. Most of the best teachers I can recall have the attitude that they themselves are still students.