Honestly I can't remember, I've been a gamer for about as long as I can remember...was probably born with a controller in my hand.
I can say that I remember getting the Genesis/Sonic 2 bundle for Christmas when I was seven. But I already had an NES before that and had had it for years (and beaten a lot of games) and I can't remember exactly when I got the NES other than that I was under 7 and it was before I got a Genesis.
So yeah, best I can say is various NES games when I was <7 years old. I specifically remember beating Mario 1, Mario 3, Double Dragon II, and a couple other games.