I was 11 years old when I got Pokemon Blue. Wanted to play the game after I read an article about it on one of the issues of Nintendo Power. I remember getting the game and the Game Boy Pocket system about a month or two after Pokemon Blue was released. If I remember correctly, I picked Pokemon Blue because of the version exclusive Pokemon. Really wanted to get a Vulpix and Meowth in the game.
My bro got it when he was 8, I was 6. So I just played it too. I can remember that time very well, because I'm dutch I couldn't read the english, and I didn't understand anything of the game. But I still thought it awesome and kept playing even while I didn't really know what I had to do. :'D I'm 16 now.
My original was Yellow, which released on October 1, 1999, I got it that Christmas, so I was 4. I could read early on, so once my sister was done, (it was actually for her, but she got bored by New Year's) she gave it to me. I wouldn't have gotten the core mechanics were it not for the anime, which my sister had watched since it debuted. My sister gave me her Blue at Christmas 2000, because she had caught 'em all. My dad got me a Red version that same Christmas, so I had finished catching by the time G/S/C released. It was cool.
Come to think of it, I could've started earlier, because I read the first Harry Potter at 3 1/2.