Personally I was 7, one month away from 8. My childhood roots have always been in video games - I spent my very early childhood years, 4-7 in France constantly playing Tetris on the original gameboy with my family via link cables. My mother, who oddly enough was somewhat a gamer, introduced me to the Megadrive (Genisis) at about 5 and I completed Ghosts 'N' Ghouls by myself at age 6.
From around 8-11 my gaming experiences went onto the PS1 and Xbox, mostly consisting of flawless FFVII playthroughs guided by my older brother and solo an co-op legendary runs on Halo 1 and Halo 2.
My gaming experience has been all over the place - I even played Diablo II regularly at the age of 9, finishing nightmare by myself. All this has led to a sad yet beautiful and undeniable link to video games. They've not only molded my tastes, but my life. I truly believe games such as Halo, Diablo and FFVII-X are what caused me to be more intelligent than classmates when I was younger. Constant reading from RPGs, and an expansion of vocabulary did a world of good for a younger boy.
I'm rambling. 7 years old is the answer.