The first game I ever completed was the first game I ever played-Super Mario 64. To this day I recall slamming Bowser into the arena spikes for the third and final time as the ground crumbled. The regular recoloring in the DS remake did not do the bout justice.
I actually didn't start playing Video Games until I was 9 and got a DS, PH being one of my first games
but I didn't finish it until like 2 years later (restarted every time I got stuck )....finishing it also caused me a HUGE sore on the inside of my mouth because the final battle was nerve racking xDD
I believe the first game I completed was Super Mario Land for my Gameboy brick. It took me forever to figure out I could jump on the Chibibos (which are now apparently called Goombos according to the Mario Wiki ._.), which I used to call parachute guys when I was young :3
Majora's Mask when I was like 12, we lost our N64 when I was maybe 6, and I only watched my older sibling play games on it. and I discovered the wonders of PJ64 around 12, before we got a wii. I thought the game was over after the first 3-day cycle.