They were joking. That was another one of FUNimation's flubs before dubbing the rest of Dragon Ball. Because Goku was so shrimpy and completely clueless, they assumed he was five years old and had Bulma say she's known him since he was five.Mystic Jack wrote:Technically he was 12, but he was a little shrimp so it's understandable,=.
Should be noted that Dragon Ball's dub pretty much fixes some of the errors made in Dragon Ball Z when they made call-backs to Dragon Ball. Goku doesn't claim to be five years old, nor was Tao Pai Pai refered to General Tao (excluding the third DB movie), and Dr. Gero is no where mentioned since Commander Red is the one running the Red Ribbon Army.