Goku’s age varies depending on the media he is featured in. In English dubs of the series, Goku is 12 years old when introduced in “Dragon Ball”. Depending on the transliteration used for his name then, he was either 12 or 13 at the end of it (18 ff common equivilents) with a birthdate of March 18, 737. At this point Krillin became 11 and Roshi turned 70-years-old. In episode 79 while fighting Dinosaur(an), Tien asks why Goku doesn’t use his full power to which Yamchas overpowers Goku with only a third of his strength and responds by saying that because Goku never trained enough even as a child he couldn’t get any more power.