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Super Mario bros 3 on NES. Was 5 years old. Me and my cousin used the game genie so not sure if it counts. Added 99 lives and the ability jump while already in air.

The first game I beat without using cheats was Super Mario Bros 3 but the Allstars version.

Holy shit. I still go through bouts trying to beat 3 whistless. 

Not sure really, it was either Choplifter on C64 or The Hobbit, graphic adventure game on ZX Spectrum - either one around 82/83.

I'm not sure, I had a number of first games on my first PC at the end of 1997:

- Need for Speed 2 - I don't remember beating it

- Screamer 2 (Racing) - the same

- Monster Truck Madness - the same

- Full Throttle - I beat it only in 1999

- Shadow Warrior (FPS) - only a few or several months later

- Warcraft 2 - I don't remember beating the campaign

- Dark Colony (RTS) - the same

Don't remember since i stared playing since I was like 4, but probably:

More than 20 years later and the game is more popular than ever, with multiple expansions.
'The Three Kingdoms' Expansion is dropping today

Adding 5 new civs and lots of free updated including unique castles for every civ etc.

PS5/Three Kingdoms Trailer

https://www.youtube.com/watch?v=R6f60UoWSag&t=52s&ab_channel=AgeofEmpires

I think technically it'd be Snail Maze, the secret build in game of Master System. Or the build in Light Phaser game Safari Hunt, though that I only did shooting close, like a meter away.
I don't remember if cousins' Videopac (Odyssey 2) had any game that you could "beat", that would've been before.
Couple of earliest SMS games were Golden Axe and Slap Shot, but those are pretty short and easy. Excluding endless games, I'd say my first real gaming achievement was Tennis Ace which has longer tournament and I also maxed stats with training which took further longer.

Same for me as I don't recall any of the Atari games I had being "beatable". 

Correction after digging a little Pitfall had an ending but i do not recall achieving it. So still SMB on the NES.

I have no idea. Probably a NES game. Maybe Mega Man 2 or something...

On consoles, Super Mario Bros. Once you know where the warp pipes are, the game gets pretty short and easy.

On PC, Dune (the Virgin one, not the Westwood one). Also here, once you know how the game ticks, which is pretty easy to find out, you can play the Harkonnen AI like a fiddle. I actually stalled a lot to find all the hidden sieches in the game instead of finishing off the enemy in their palace.