You're right I think it has to do with growing up really. A lot of people who are into the COD stuff right now perhaps weren't into FPS growing up. I mean video games used to be for shut ins but now it's socially accepted. You'll see hot girls playing it, I don't know if they really enjoy it or are just trying to relate to men. I think my girlfriend did legitimately like one FPS I got her into.
But when I was growing up I gamed a lot and the only games I ever played were RTS which focuses on tactics and quick decision / precise movements and FPS. So I naturally am a beast at these games.
My first multi-player console game was Socom US Navy SEALS. This was the first console game to use the playstation microphone and people actually used it for tactical talk back then, I made a lot of friends with that mic.
Before that it was playing any free online FPS I could download. Before that it was Wolfenstein 3D and DOOM. So I think the really good people like ourselves just have lots of experience and are more intelligent as far as tactics go from experience.
Personally I don't really like call of duty that much I heard it described as a twitch shooter and that's pretty correct. Though I do like some twitch shooters like open arena and Quake... I don't know what it is specifically about COD. My favourite times were with Socom that's for sure.
I don't think video games have been for shut ins since the early-mid 1980s. They were marketed as entertainment and arcades were already hugely popular as a social entertainment pasttime. Apparently 30% of homes in the U.S. had an NES (
source, pg 26), this is compared to 65% of U.S. households that currently play video games (
source). I'm sure the average person had the chance to play video games often even if their family didn't own one themselves, it has long been socially acceptable. I think the difference is the attitude towards female gamers now that we have internet and online gaming, the global perspective is becoming aware of the true demographics.
As for CoD, yes it is a twitch shooter. In the early years however the popularity was attributed to the WWII/war appeal, then it became known for the competitive multiplayer, by 2006 the CoD series captured the market and became a cultural icon. This was all before Modern Warfare, the game arguably responsible for the "twitch shooter" label.
2008 is where I personally switched to Battlefield, I could tell the series was getting stale by that point, at least the multiplayer aspect.