A Commodore 64
I was nine when I got a Commodore 64 for Christmas. I wasted no time exploring what it could do, which mostly meant typing in programs, changing them, and seeing what happened.
One of the first things I remember programming was a tiny green, pixelated hot-air balloon that I could make rise and fall on the screen. It was simple, but I could type something and watch the machine respond.
That was enough. I was hooked.