Use it or lose it.
I saw a great video the other day about perceptions of ageing. People in their 20s and 30s were asked what "old" means to them then asked to demonstrate how old people might do things, including walking, texting and exercising.
They're then surprised to be introduced to people in their 60s and 70s who are dynamic, fit and vibrant and who easily manage all the physically challenging things the younger people can do. To see the video, search for "disrupt ageing".
There's no doubt our idea of old has changed. Sixty is not what it used to be. Most people in their 60s and 70s don't consider themselves old and most are active and vibrant.
Many are also still working in some form, as the nature of work changes and the idea of retirement at 60 loses its appeal. Not everyone wants to spend their time in inactivity and the baby-boomer generation is far less inclined to sit quietly in the corner.