My parents allowed minimal television, but it never seemed unfair because they incentivized the time we WEREN'T watching TV. I assume you could apply a similar method to internet/video game time.
Each week, we were given little bingo tokens that represented half an hour of TV time. I think everyone (parents included)only got 5 tokens a week. If we had a specific amount of tokens remaining at the end of the week, there was some sort of reward - dinner at a restaurant, a movie rental (christ, I'm really dating myself), or even just getting to stay up later than usual. For your kids it could be a new game they want, or a later curfew. The less screen time they use, the better the reward.
When other kids were talking about some show they watched the night before, my brother and I didn't care, or feel excluded, because we knew we were getting ice cream on Friday instead. And since the more tokens left over at the end of the week, the better the reward, my brother and I cooperated on what shows we'd watch, and ended up spending more time together because of it.
A 17 year old is almost an adult, and provided he/she takes care of homework and chores, and gets plenty of outside activity (and sleep), should be allowed a more freedom than a 9 year old. But for both, offering incentives to put a limit on the screen time, and finding ways to encourage them to interact with each other (and you) while enjoying video game/TV/Internet time is pretty valuable.
I almost never watch TV as an adult. And it's not because I'm a snob about television, it's just that unless it's a show I REALLY want to watch, and can watch with someone whom I can discuss it with afterwards, it's just not that interesting.
Thanks mom, thanks dad.