Sooner or later it will happen to all of us. The older guy on the mat that you were reluctant to train with because he was too slow or too stiff or too brittle becomes you. What should you do? Leave the mat and take up knitting? Doubtful. Instead, like all the adjustments you start to make you need to find a way to continue doing what you love; perhaps, a little slower or with a little less power. Or perhaps less often or with more time spent sitting seiza on the back of the mat.
What is the role of the Sensei in helping you make the adjustment? Does he expect you to fly around the mat as always or does he (heaven forfend) create an Aikido lite class– one with little or no ukemi and techniques more suitable to your current ability? Whether and how to create this environment are topics to be explored in my blog.