Goku is 36 years old.
Goku was born 30 June, Age 737 which would make him 36 by the first episode of Dragon Ball series. He was found as an infant on Mount Paozu nine months after King Piccolo’s reign of terror and martial arts tournaments had ceased during this time period, it follows that he’s 9 throughout most of the original Dragon Ball anime and 10 throughout all of its sequels and movies.