I'd much rather learn from someone who knows what he/she's doing than someone who is healthy and fit and doesn't.
My instructors have their health issues but they can teach and perform Aikido around their health problems.
My sensei is an older man ( I can't call him old with a straight face, though he's in his mid 60's), who can manhandle me with complete ease.
There was a period a few years ago where he was recovering from a stroke. His classes dwindled in size , and I'm not entirely sure it wasn't because students were turned off at his evident physical deterioration.
It ended up being mostly me going to his classes. During that time I learnt a ton, more than a ton. I learnt that in spite of his problem he could teach AND deliver, and protect himself and me.
He eventually recovered back to normal, though for a while people acted as if they did not believe that was possible.
It's good to know that Aikido still works.
I even heard that there was an Aikido demonstration from a wheelchair during the last expo, and people have practiced with blind students before.