"Walking is martial arts"
What this means is that no matter what sort of physique you have, you can do Aikido. Whether you are rigid or soft or thin or heavy set, you can do Aikido in your own way as long as you find the way to move that's most natural for yourself.
In other words, you must continue to improve your physique so that it is not forced but natural. You could say that this is the unrelenting struggle of shugyo. So, if you develop the physique that is best suited to your age, you will be able to do Aikido forever, no matter how old you are.