A third theory often rounds out the activity and disengagement theories, and it's called the continuity theory of aging. Of course, most people want to continue aging but without getting "old," and this theory is kind of a "keep on keeping on" complement to both, allowing people to just keep doing what they're doing without necessarily ramping up activities or slowing down life considerably [source: Atchley]. Continuing on without any activity or interactions isn't good at any age, though, and people of all ages and physical capabilities need some type of socialization and movement to keep going [source: CDC]. But it isn't necessary to radically change what you do in order to sustain or improve quality of life.
All three theories leave room for passive-active aging depending on the person, and it may just be natural to slow down or stop certain activities based on what your body tells you. Engaging and withdrawing isn't so much activity or inactivity as it is making the most of time with true pleasures and best companions instead of forcing interactions for the sake of being "active." Some enjoy social activities over physical activities and vice-versa, so an "activity theory" for you might look entirely different than that of another person. Overwhelmingly, though, studies do show that being active and social reduces depression, increases well-being and adds years to life [source: CDC].
Joining a senior center or jumping out of a plane can greatly improve your level of happiness, if you see yourself doing either or both. But if not, do what you already enjoy or take up something you've always wanted to do, whether it's joining a walking group or putting together a meal with friends. Moving in order to live longer is important, but getting excited and living to move is a lot more fun than any theory about how to do it.