If it makes you feel better my little brother has also been playing halo and counter-strike since he was 4.
He used to just run around looking at the water and grass on halo, we kept letting him play thinking thats all he was doing and next thing i know i walk into the room and he's throwing plasma grenades at grunts and laughing his *** off as they run in circles sreaming and then blowing up.
He got pretty frustrated with the controller so i tought him how to move on the pc (i only told him "w") he picked up switch weapon, pick up weapon, jump crouchall on his own. It was crazy to see how he managed his tiny fingers to hit multiple buttons at the same time. (he was really frustrated, for example, on how to jump and crouch onto crates and such because he had to hit w, space, and ctrl at the same time. But he got it down. And after he saw me and my cousin playing cs, he got intrgued in that as well. My cousin showed him how to use the mic and man was that fun! He always refered to guns as a 3-1 or a 4-3 haha. It was crazy, pretty cool but sometimes hard to kick him off when i wanted to play. He got crazy into toy guns after that and was always dressed like an "army man" or a cop.
Anyways my parents finally realized he was getting addicted since thats all he wanted to do and had him cut back, and now he's 6 can't play any "M" games. I keep getting games like George and Over the Hedge and Chicken Little and he's getting good at them, figuring out the little puzzles and such which I think is better for him. But he also now has a stuffed animal named Zack that he just brought to the movies with me and my gf and he held it up so he could watch it too. She thinks it's "so cute" while I remember the soldier that once was.