Well, toward the end of Dragonball, Goku was a teenager and looked pretty much as he does in Dragonball Z, so it looks like they have skipped around pulling different elements from different parts of the story. Goku was a kid when he fought King Piccolo, who is plainly the main villian of the Movie, but he was a teenager when he fought Piccolo Jr. (the Piccolo who eventually becomes good and goes on to be one of the main characters of Dragon Ball Z).