What would you do if a 50 year old Shihan came into you dojo and wanted to take part in a beginners class?
We say "of course" and ask he he or she would like to teach. Friday night is our Introduction to Aikido class and on one occasion a shihan flew in for a weekend seminar, dropped by the dojo to say hello, and ended up teaching. It seemed he really enjoyed showing people how to do their first low forward roll and it was a great class.
I don't think we have had an official shihan on the mat as a student in the beginners' class though we have had 5th dan students including one that was shortly afterwards promoted to 6th dan meaning that we must have taught the right stuff.