I've heard it said there's a statement in the Gemara to the effect of "Someone is still in full good health who can take his shoe on & off without falling or leaning (i.e. balancing)".
Where is it?
ילד עד כמה אמר רבי אלעא אמר רבי חנינא כל שעומד על רגלו אחת וחולץ מנעלו ונועל מנעלו אמרו עליו על רבי חנינא שהיה בן שמונים שנה והיה עומד על רגלו אחת וחולץ מנעלו ונועל מנעלו
Until when is one considered a young person? Rabbi Ela says that Rabbi Ḥanina says: Anyone who is able to stand on one of his legs and remove his shoe or put on his shoe is considered young. They said about Rabbi Ḥanina that he was eighty years old and would stand on one of his legs and remove his shoe or put on his shoe.