A lot of this is denial - about aging or slowing down physically.
All of age in different ways. Joint and muscle issues begin affecting some people very early in life. Others feel it much later, and some escape most of it for years. Nevertheless, activities that are comfortable for people in their twenties or thirties become more challenging a decade or two later - brisk walking, running, climbing stairs, playing tennis or racquetball, and hard exercise, for example.
The point is that even when people - at whatever age they are now - can do activities relatively easily (maybe not as quickly as they could at an earlier age but still relatively easily), that may not always be the case. Therefore, making accommodations for the future in terms of a less robust physical ability can make a lot of sense.
That said, where does one start, and what areas should someone focus on?
Mobility is a big area of focus - anything having to do with the joints or movement. While activities involving the hips, knees, fingers, wrists, hands, ankles, and other joints in the body - along with the muscles, tendons, and ligaments that control their use - may not be a major concern at this point in a person's life, that may not always be the case.
This is the time to add universal design elements to a home - for the future when activities may become more challenging or difficult and to accommodate visiting neighbors, friends, and relatives.
This is the time to begin looking at ways for widening doorways, eliminating steps (into the home and inside it), changing out flooring so it is more hard surface, changing out door handles and cabinet pulls, making sure faucets and appliances can be used efficiently, and widening or eliminating congested areas in the home where passage or cabinet doors open onto each other or walls or other features (such as islands) create a narrowed passageway.
Rather than wait until an issue arises with mobility - from a household member or a guest - now is the time to begin addressing them. When universal design measures are used to accommodate changing or more limited mobility patterns, the person with the issues is able to function more easily and successfully in the space, but the features fit right in and do not look any different from what is there now - except by function.
Another important area that comes into play as people age is sensory. This involves such activities as eyesight, hearing, touch, depth perception, stamina, coordination, and balance.
Again, by addressing such issues now before they become major concerns, people can function quite well and safely in the future when faced with limitations.
Now definitely is the time to begin addressing the possibility of future needs, and when done with universal design treatments they will not be noticeable to people unfamiliar with what has been done, and they will enhance the functionality, safety, and value of the home.