It's true that some people (i.e. me) slow down as they grow older. I specially notice this in the mornings when it takes me longer to get up and dressed. But I'm not sure this only has to do with physical action.
What I find is that I'm constantly enlarging my mind, my thoughts, my heart. The latter has to do with children marrying and producing off-spring, my husband and I making new friends, and for me, entering into writers' communities, and just generally meeting people. As to my mind, I find I spend more time musing and thinking things over than I ever did before.... It's like an expansion of consciousness, if you will. So maybe if I'm putting so much energy into the more interior matters of being, it doesn't really matter if I'm physically slower than I used to be. Would you say?