Your joints won't have much motivation to strive for full mobility if they're never called upon to do so. Without being fully extended, they'll start trying to get away with as little work as possible.
Stretch out every day. It doesn't have to be an extreme stretching marathon -- just spend a few minutes in your spare time manipulating your fingers, wrists, arms, ankles and anything else you plan on using for the rest of your life. Each stretch can be as short as five or ten seconds. The point is just to remind the joints that they do still in fact move, even if not very often. Try to move your joints through their complete range of motion, as long as it doesn't cause you pain. You can do this by using sturdy household objects to provide support while you gently turn, stretch, twist and limber up.
If exercise is on your schedule, stretch both before and after the activity. Not stretching before physical activity leaves you more susceptible to injury, which could drastically reduce your mobility and make matters much worse. With regular stretching, your joints won't just retain mobility -- they'll grow more limber and show you results you may have thought were no longer possible.