I'd like to start by saying that I'm not a parent, ut so it's all clear..
I'm 26 now and I grew up in the days of the ZX Spectrum and BBC Micro (Nearly missed them though!)
From the age of about 4 or 5 I was introduced to gaming on the spectrum, I used to spend a lot of time playing things like the original turtles game... But I also ended up playing things like Lord of the Rings.. basic text adventures... at 5 years old my understanding of the world increased... I learnt about north, south, east and west and what they meant, I learnt new words and it sparked my imagination... It's not the SCREENS that's the problem, it's what's on them... At the age of 6 I wrote my first program on the spectrum because I found the spectrum manual on the desk one day with a big title of "make your own games".. I admit, they programs weren't very good, but it was the fact that I did it.. I learnt a lot back then (and probably how I've made it into a job as a programmer now)...
Probably around the age of 8 or 9 I got a computer of my own in my room! It was a commodore amiga A500+. Puzzle games like Monkey Island helped my brain to develop with logic skills...
But anyway... to the point I'm trying to make before I get lost in a world of nostalgia!
"Screens" and games aren't inherently BAD things... It's all a matter of balance. If there is genuinely a learning experience through what they're doing on a computer, leave them to it for a bit longer. But if it's just for games make sure they have some more stimuli than just working out how to jump across platforms. Take them out to see the world, play with them in the house, above all ALWAYS take an interest if they're learning and give them encouragement.
In conclusion: Games, computers and TVs are neither Bad nor Good. It's the parents that manage these things that makes the difference to the child. The problem is, parents don't want to accept responsibility and like to blame the easiest thing. If you think games are having a bad effect on your child, don't STOP them from playing them, they'll only scream and yell, what you have to do is make times that they're not playing it fun as well so that the have a choice. If you give them no feedback when they're playing they'll go back to the only thing that will give them some kind of feedback and be mentally stuck there.
That turned into a much bigger post than I meant it to.... I was just gunna write a couple of lines.. lol