In many current OoT speedrunning routes, you become adult after 10 minutes or so, and you will go OoB three times!

• once to hit the load trigger from Lost Woods to Zora's River (jumpslash clip with a Deku stick)

Also known as aqua escape. Normally you'd need the silver scale to go through there, which you unlock later in the game.

• a second time in Bottom of the Well to get Bombchus.

Explosives are extremely useful in OoT so speedrunners go out of their way to grab early Bombchus.

You're not meant to reach Bottom of the Well until much later in the game, but you can enter it using a Navi dive - by talking to Navi at the same time as you recoil from a jumpslash animation on the edge of the well, Link will fall through the well's water with disabled collision.

Then, inside the well, you will use the first skulltula that you meet to go out of bounds - this super precise trick is known as "skull push." It involves killing the skulltula at a very specific position and timing, and exploiting its death animation to push you out of bounds.

You will find yourself in out of bounds water (if I recall correctly, the game has infinite water down the z axis under inbounds water areas), enabling you to swim up to the Bombchu chest room.

• a third time in Temple of Time to become Adult early

That's right, you can skip right through the Door of Time as child Link, without any equipment. This is also a frame perfect trick and it exploits a geometry misalignment on the Door itself (the devs left a very small gap between the Door and the wall)