Obesity, Diabetes Type II and so on
The first thing that comes to mind when I read about increased numbers of young people (well, under 25s anyway) spending more of their time indoors playing computer games are the side effects of such a sedentary lifestyle: obesity, diabetes type II, tracksuit/pyjama wearing [1] and so on. In other words, the ticking time-bomb is being magnified.
For all of us in our 40s and 50s, these won't have a positive effect on us when we come to retirement age in 20 years' time. But this is just me being selfish.
That being said, I played a lot Quake, Half Life, Team Fortress, StarCraft & Civilization in my 20s and early 30s, so I'm hardly one to complain. Computer games, *when played with others*, are immensely satisfying and very good for social contact. So they can have a positive effect on our mental health and a detrimental effect on our physical health. I'm still waiting to find railguns that float about a metre off the ground.
[1] I don't mean to come to come across a snob here. If you're going to stay in all day anyway, then putting on the easiest and comfiest clothes seems like a good idea, more of this notion.