They need to understand the principles of Aikido, and be able to demonstrate them on their students, so the students can feel the technique. I believe O Sensei was teaching well past the prime of his youth.
Some people have difficulty learning from someone who doesn't fit the sterotype of what they want to be from taking this class (so they cannot study under eldery/overweight/female etc instructors). While I think that kind of thinking should be modified, luckily there are enough dojos around that most can find someone they like; after all, some can't learn from those who yell, some from those who talk too much, or talk too little, or use Japanese, or etc. A place for us all.