20.
First game is a tie between Duke Nukem/Breakout in MS-DOS, and failing miserably at the original Mario Bros.
My first actual memory of enjoying a game and grasping it was sometime when I was still in my single-digits, but I fondly remember moving into a new house, no furniture, and laying on the carpet with a tiny TV and my SNES playing Super Mario RPG: Legend of the Seven Stars.
Although when I became an alert gamer coherent enough to make sense of all the games I played, I have no idea.
Also, sorry older folks. If it makes any of you feel better, I have no clue what any of those experiences are like. NES was really my intro to gaming.