Because after he was a grown man, and beaten Majin Buu, an old enemy got dragon balls and turned him into a kid again
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Empire pilaf (i cant spell it the little blue guy from when goku was a kid) uses the black star dragonball to wish goku was a kid again. this happens in dragonball gt not z
Goku turn into kid Goku in episode 1 or Dragonball GT because Emperor Pilof accidentally wished for Goku to be a kid again.
Dragonball GT Episode 1: Secret of the Dragon Balls
get all the way to gt saga and finish it then go to duel or what ever and go to gt kid goku and go to ssj4 goku or gogeta ssj4
The last episode of DragonBall GT and the DragonBall GT movie, "A Hero's Legacy."