Obviously, the reason for this is that stupid thing called life getting in the way. It's not that gamers want to stop, it's just that jobs, school, relationships and babies get in the way. 48 percent say they leave because they get too busy and 40 percent say they simply got bored (no further data there, but that's worth looking into). There is also talk about how controllers have gotten too complicated, which Nintendo is currently attempting to tackle through the Wii. We'll have to wait and see how the Wii control concept plays out in the long run. The article loses steam toward the end and saves itself by bringing up the fact that although older gamers may not be on consoles anymore, they probably spend 40+ hours in front of a computer and the "casual games" market isn't hurting from that fact one bit.
The whole article is a little silly in its statement of the obvious: As we age, free time dries up. We don't watch as many movies (film industry), we don't go to as many concerts (music industry) and we don't play as many games. But the human race continues to breed and multiply, the next generation will replace us and pick up the consumer slack. We also have an issue with this "problem" being at all true to begin with because the average age of gamers, according to the ESA, continues to go up.