What's the earliest you've gotten a shiny before?

[Lysander]

Sentret on Route 29 in Soul Silver, not long after getting Poké Balls for the first time. And that's why you should never level grind before getting them.

 

[Big Baz]

I once found a Shiny Budew on Route 4 in X, a little after the first Gym.
I was actually hunting for a 1% White Flower Flabébé encounter to use on my team, but I decided to use the Budew instead - if I'd spent so long looking for a regular Pokémon that I found a Shiny first, it probably wasn't worth the effort to keep looking.

 

[Iceshadow3317]

The earliest I've ever gotten a Shiny was in... I can't remember if it was Sun or Ultra Sun, but it was one of the Alola games. And the very first encounter I got that I was able to catch ended up being a Shiny Pikipek.

 

[Palamon]

One time, I found a shiny Torchic while playing a new save file in Emerald while my parents, siblings and I were at the mall.

...And I didn't save the game.

 

[Sniwott]

I've had a couple of shinies that I obtained early in my games :P

I've gotten a Zizagoon in Omega Ruby, before fighting Roxanne on the route between Rustboro and the cave, thanks to the Professor Oak's Challenge I was doing. I also got a shiny Machop during my Platinum Professor Oak's Challenge, on the route north of Oreburgh with the Ponyta... let me tell you, that section was a slog lol

 

[RadEmpoleon]

One time I went hunting for shinies in a Friend Safari. The very first encounter was a shiny Pawniard. I almost screamed, if not for me being in the car with my family lol

EDIT: reading through the responses, I realize this is supposed to be the earliest shiny as in earliest in your journey, not the quickest shiny hunt. In that case, I never found any shiny Pokemon until post-game.

 

[Diamond 97]

I found a shiny Scaterbug in Violet Version fairly early on in the game.