My kids, all three of them, were pretty capable on the computer by 3. My stepson (one of the 3) just turned 4, and he can navigate and play just about any game you throw at him, computer or console. I just upped my 6 year old daughter to a 22 WS for her youtubing and cartoon network/adult swim games (sick huh?). My 11 year old destroys at any game you throw at him, after being Bioshock 2 and Crysis within a week, I just showed him Quake III Arena. I'm pretty sure he could rival Fatal1ty at age 11. No kidding, kid is a natural sniper, now just gotta make sure he doesn't turn to real guns......
I'm a total geek and a former network admin tho, they have always had computers around them.