Neverending training.

From a young age, Son Goku was raised on martial arts. His adopted grandfather, was a well renown martial arts practitioner, a student of the legendary Muten Roshi.
Grandpa Gohan taught Goku the very basics until his untimely death. But Goku took his lessons to heart, and kept up his training. So that when Bulma met him, Goku was already stronger than most adults, and that was just the start of it all...